Condemnation before the music is even written for the film seems a little premature. I have faith in Pixar and Mastodon to do something tasteful, even if it does play on tropes (e.g., showing an outcast that listens to metal actually being a cool guy, going on a rampage to metal, scary kids with metal, etc).

For the record:

Mastodon previously scored Jonah Hex. While the movie sucked, the score they provided was generally atmospheric and instrumental (not even instrumetal for the most part!).

Their newest album, while it has heavy songs (like the previously linked song), also has songs like this (www.youtube.com).