Of course it is, as I previously mentioned: Tumblr works as a friendship that starts with a common interest, it's a mutual interest that needs to be born in order to build something up. Bust just because you're nicely reblogging stuff, commenting or similar doesn't guarantee people will be as equally as interested, that's why focus on it being genuine, and not a shallow relationship to obtain something in return. Aka an old version of the sub x sub.
That interest can be anything, "Art" can be a fandom, "webcomics" can be a fandom, "Original character" can be a fandom, literally anything. The thing is that you have to engage, you have to go back to your early stage of being that kiddo who just got access to the internet and wanted to interact, consume and engage with everyone and then someone would come around and be interested in you as well.
There are a lot of artists who post original content too, some have been there since I was 13.
really the two keys to posting art is 1. knowing how to use tags, and 2. having a dedicated art blog
This is from cozylittleartblog
tumblr only tracks the first 20 tags on a post, everything after that is Just For Funsies. tracked tags will show up in search, both for the whole website and on your blog.
if you're not sure what tags to use, see what other popular posters are using! i think i mentioned this in my tumblr guide post, but generally you want to tag fandom, characters, ships (if applicable), and any AUs or similar things. oh, and content warnings if those apply, too. then if you want you can try stuff like mediums and other general tags, that'd be your #art, #illustration, #fanart, #fanfic, #digital art, #(program you used), etc etc. you don't really have to use those, but if you want to keep your blog Super Extra Organized or if you're posting original content with no fandom tags to lean on then they are good options. (you can use things like #ocs, #original characters, #character design, #concept art, and genre tags for those!)
If you consider that you should gatekeep your feelings of what you like because you don't want to """""do free promotion"""" then... you won't succeed either when it comes to other people's work, other people's content be it they are new, veterans, unpopular, popular, etc.