It can never be straight up mimicry, for the sole fact that each band has different members, who in turn have different musical prowess. Everyone is influenced by someone, but most bands gather several influences into their music. Muse is bad because they basically just take Radiohead's sound, and add their own bits. It isn't anything new, and thus that's why it gets the outcry of Radiohead fans.

It's not like they're the only band ever to do it, though. There were countless Led Zeppelin imitators. All it does is place them in the "Non-Original" group instead of the "Original" group. Only so many bands can take their influences and pioneer a completely new sound. In the past fifteen years, there have really only been two bands to create a new, unheard of sound - Radiohead and Mike Patton with all of his projects.