I think it's when you take from somebody else without permission, especially if you take and then try to pass it off as your own work. So basically "theft" I guess is what I'd also consider "cheating".
I don't see a lot else as "cheating" really. AI is like... well, fine, if you're using it from a database you have the rights to use all the images from, tracing over 3D, or even tracing over photos is like... yeah, fine, if you have the rights, go for it.
Like I don't trace 3D models for characters and faces, but it's not because I think it's "cheating!" it's because I think it looks static and "posed", it tends to create a lack of variety of physical builds, and setting up the models takes longer for me than just drawing the figures. If somebody else wants to use that approach, that's their choice, they have to deal with the downsides of that approach as well as the upsides.
Saying other creators are "cheating!" for using time-efficient methods is often an excuse people use for not branching out and learning to use new ways of working to compete. The audience doesn't owe a creator their attention for using a method that takes more time, like painstakingly constructing 3-point perspective from a grid on all your backgrounds, working with trad media etc, and the people who sensibly choose to use timesaving methods to make pages faster to deliver more entertainment to their audience in a competitive market, and so build a bigger following aren't "cheating".
I think the only thing I can think of as "cheating" that isn't actively theft would be something like pretending to be from a marginalised group to get boosted by Tapas. Like if somebody white like me submitted to Black History Month, that'd be cheating. That's an awful thing to do (and if anyone ever found out...hoooo boy, you'd be in for it, so it'd also be an extremely unwise thing to do).
That said, because I think this is an important addition, I don't think putting diverse characters into a work to appeal to a modern audience, and building a readership thanks to doing so is "cheating", even though I do see people implying or outright saying it is. People aren't stupid; if you put diverse characters into your comic without doing research and it comes out not feeling genuine, or is insulting, it'll have a negative effect, so like any approach, it has upsides and downsides and isn't just an easy "get readership fast with this one weird trick!" thing, so it's not cheating.
Using sock puppet accounts or bots to artificially generate the appearance of engagement. That's definitely cheating, and against site rules. That's something I'd call "cheating".
Sub-for-sub is kind of cheating... but also it's such an ineffective approach to promo, it's like a form of "cheating" that's actually worse than the alternative, so I don't know if I'd even call it "cheating". Although... the phrase does go "cheaters never prosper" so maybe that fits? It's a quick way to boost the numbers without the hard work of research and improvement to build an organic following of people who are genuinely entertained... It's kinda cheating, and like most forms of cheating, it doesn't really pay off; you just end up with a comic with enough subs to unlock revenue...but not enough activity to generate any. 
I dunno, I can't think of a way of cheating that's actually cheating that wouldn't almost certainly have a worse outcome than just doing your best to make good work and advertising it honestly, but paying attention to trends in audiences, design and marketing to make the most of them. 