No, I don't think it's appropriate at all, not while the fest is running. After the reveal, when everyone takes their fics and goes home, so to speak, the fics are open for con crit and reviewing just like everything else. But IMO, while the fics are still considered gifts, negative reviews and con crit is completely inappropriate. Especially if the fest is anon and the writer can't say anything to defend him/herself if s/he is so inclined. The fics will be out there for the rest of eternity; if someone really feels the need to give them negative reviews, they've got forever after the reveal.