Honestly, I don't care much as long as it's, like other people said, appropriate to the faire itself. At the Texas rennfest, which does multiple themed weekends (celtic, pirates, hallow's eve, fantasy, warriors, etc), I have seen multiple just bland homestuck, doctors or star trek cosplays, which I don't particularly like, but it doesn't butt frustrate me.
I'll take any at least appropriate-looking cosplay, however, since I'm aware "Renaissance" in "Renaissance Faire" is a term to be taken loosely, and is not solely about the actual Renaissance period, which is why I don't get ass-angry about pirates, or fairies, or elves.