To address the OP's question: It really depends on the artists being given the tribute. In the case of Elvis, yes, the look and mannerisms were part of his original appeal, and thus, would be critical. In the case of other artists, particularly bands that were known for things other than their looks, it might not matter, so much. The larger the original band, the less-important the look. It might be more important for a Police tribute band to have a blonde bassist, with somewhat spiked hair, than it would be for a Yes tribute to have personnel who look like the original band. In the latter instance, sound is the most important ingredient (indeed, any prog-rock tribute is going to be primarily sound-focused, although an exception might be a Rush tribute). I'm doing my best to get a tribute going for a certain keyboard-based trio, where the singing bassist is somewhat stocky, whereas I'm on the slim side. What matters more is that I can play and sound like him.