I don't know if the mods keep track of that directly or not. Sometimes it seems like there a lot of alt accounts on imgflip, so they do seem to let it slide.
But if someone were rigging it at that level, and really bad memes were making the front page under the same name constantly, then I assume it would get to the point that the mods would do something. At least I hope they would.
What I was thinking is that it's a bad idea because it's just so empty - part of the fun of making memes and comments is getting that feedback of what connects with people. If you were just upvoting all your own, you'd miss out on that in a big way.