There are signs for people who worship devils. 1- They have skeleton logo in thier outfit. 2- They tend to wear black. Smokey eyes... Black nail polish...and so on. 3- People sometimes say that rockstars are Gothic of devil worshipers.
Otherwise, I really don't know about Avril, but she's got one of the signs. BUT IT IS NOT A RULE. SO DON'T HOLD THAT AGAINST ME!
She might be and she might not be. Honestly, who really knows. It's not like she would tell the entire planet "Hey! I worship the devil! I think u should too!" I believe she has sung some Christian songs. Anyway, just because she wears dark clothing, wears skulls, and swears just means her morals are in the current times but doesn't necessarily mean she worships the devil. It has zei in numerous places that she is a Christian, which means that she worships God, the opposite of the devil (just for those of u that don't know). I guess no one can really know when it comes down to it. That is, unless u have telepathy. =D