Carnival is not against catholicism. In fact, carnival is a "catholic thing", protestants do not celebrate it. For catholics carnival is a chance to party before the "cuaresma" (I dont know the english word), and that's the 40 days of easter that you are suposed to be respectful and doin sacrifices for yourself (i.e. stop drinking or smoking).
I am Catholic but I dont often do this, but also I'm not really in the carnival thing. I don't like wearing costums... je je

I hope it answers your question!!

CHino