I try to comment as often as possible on comic pages, but it really depends on the subject matter of the page. I especially love when my comments are replied to as well :3!
I don't comment very often, though I'd like to start commenting more to try to become a bigger part of the community.
I used to hardly comment on stuff at all, but I do it more often these days - I mean, I know how happy I am when people comment on my stuff.
That said, I'm less likely to comment on something when they a.) are super-popular and already have tons of comments, or b.) I don't have anything to say other than "Aw, cute! <3".
It also depends on the time of day the comic updates/when I read the update - if it's at a time of day when I'm properly awake and alert, I'm more likely to comment. If it updates just before I go to bed, I probably won't, because I don't make much sense when I'm tired.
I comment sporadically much like how I reply to topics on here.
I try to only comment when I think I have something constructive or interesting to say, but lately I've been thinking of just throwing in my two cents on more comics in an effort to encourage artists and be more into the community. If you flip through series there is definitely a contingent of people who post on a bunch of comics consistently and that might not be a bad way to follow.
I do think some comics are more conducive to comments and discussion than others and it has little to do with whether or not the comics are engaging, just that some have more room for talk. There are plenty of great comics on here that don't have a huge amount of people talking about them.
I'm actually the same way as @joannekwan in regards to posting more in lesser known series. After reading these posts I wonder if just dropping a word or two would make an impact on the community. I mean I know I love reading the few comments I get.
i love GETTING comments. like they really help pep me up to go on. buuut i dont actually comment on other peoples comics that much. ima terrible person. usually when i do i try to say somethin more than just "cool" or "haha". but usually all i can think of is either "cool" or "haha"
i usually read a lot of popular comics tho and when i comment i like when the creator answers back or somethin. but they usually dont on the popular ones since they get soooo many comments
anyway yeah im pretty sure this is the same as what everyone else was sayin but i felt like sayin it too i guess
¯_(ツ)_/¯
I reply when there's something about the comic that I like or it has made me think about something related to it. I try not to make unsolicited advice about their comic, no matter how good my intentions are because that could be kind of sensitive. So yeah, if it in the description of the comic is asked for advice and pointers then sure, I'm game.
And it's sooo much fun to get a comment on your own comic so I try to think about that, too. We don't get paid in money, we get rewarded in likes and comments here 
I always comment when I have something to say. It heavily depends on my mood as well, so it's as random as my personal ups and downs. I personally like reading comments so if I have something to add, that feels like a natural addition rather than just some verbal diarrhea, I will jot it down
I like to comment in the comics I'm reading, specially when I've talked with the author and i think well, if I like to have comments, then I should do the same aswell. But in popular webcomics I don't really comment xD
I really only comment for two reasons.
To compliment the artist if I really like how something looks
or To make some kind of joke or (and this ones obvious) to give some clever (or sarcastic) commentary on the comic.
I'm not really someone who likes pointing out mistakes in anything (especially grammar since English isn't always their first language) unless it's something major.
But I tend to do the first two a lot...
I try to comment on comics whose authors i know, and i make an effort to say something if I find new series I like. The rest kind of depends on whether I've got anything to say, really. Should probably do it more, really.
Not that often really, mostly because:
I often browse other people's comics while on my phone and, as a creator, I haven't found the app very good for monitoring notes and messages I'm receiving at the same time. So right now, I use the mobile desktop version for both, which makes leaving comments absolutely horrendous.
I sit quite far away from my laptop when I'm at home (like a good 4/5 feet) so I'm writing things out actually using my cintiq, which is waaay more trouble than it's worth with all the typos I make...
If I'm late to a page being posted you can guarantee someone's already said what I wanted to say.
I never have much to say besides 'neat' or 'OH SHIT'
So, basically, if technology can rise up an cure my ineptitude we'll be cooking with gas.
I do make an extra special effort for new comics or comic creators in crisis, though. I want to help them and give them some encouragement, but unless they make some noise I'm not going to know they need it. Stupid, ey?
I try to comment as often as possible on people I'd like to get to know better or comics that are starting out and try to be encouraging. Dunno if it would be considered spam. Mostly the thought that crossed my mind at the moment (usually pointing out the art style I love or tidbits, maybe ask a question about characters, maaaybe go "oh no!" only if it's a very impacting scene, or...make a joke/pun and I know the latter one is a hit or miss with some people, haha...)
But the goal is mostly to tell the author: I'm reading this, I'm enjoying this, I'm active. But I'm not expecting a reply all the time. If not, I vote up a similar comment I agree and leave it at that.
I'm glad they have this feature because I do this often when I don't leave a comment.
And don't worry, I don't think any creator would consider it spam unless you're posting "first" or some nonsensical gibberish. xD
I don't comment very often, when I do it's usually to let the creator know what I really liked about that episode, or, if that's already been said 29385235 times over, I'll try to be friendly and say something related to the episode, and I'll get a friendly convo going 
If I have something to say, to commend, or comment, I often do. I reply to most of my conversations as well.
I don't comment all the time though, because if I don't have something to say, it doesn't really do too much.
Also, I personally do not like posting vague comments like "so cool!" "That was awesome!" or "that was funny!"
I despise the word "lol" on many levels.
So I try to be clever or intelligent with my comments. If I don't have anything, I give it a like and move along.
Just realized I'm a total hypocrite because I love receiving comments on my work, but I don't leave many- I'll have to work on that!
A lot like you said- if a comic has a lot of subscribers already I probably won't comment despite its awesomeness levels- but if something is both brilliant AND under-appreciated I'll write something.
I don't comment much either, usually because I'm either catching up on the series or because others have already said what I think, I do try though. I've only just started out on my comic and all the comments given have boost my confidence so much, so yeah, they do help a lot and I want to comment more even if it's to make the artist happy.
I don't comment as much as I would like to since I don't always know what to say, but I do like leaving comments on pages that have something funny or adorable or even an expression a character made that I just loved.
I also tend to comment when I feel a kind of connection with a character.
I follow some stories that I just LOVE but I still don't know exactly what to say, or the updated page doesn't leave space to comment.
Most of the stories I follow have many comments per update, so I guess I'm the one who just can't think of anything...
(Some other times I see all those comments and think: -sigh- "someday " 😂