Lot's of Freddie Mercury's stuff has too much vibrato for me. Progressive Rock, as a genre in the 1990's/2000's almost required too much vibrato out of the singer. Meatloaf. David Bowie often went too far. Aaron Neville.

I guess for me, any time it seems that it's theatrical - that is - forced and pretending to be a character of some type; or just to be different for the sake of different-ness, then that's too much.

It's a shtick instead of decoration that adds beauty to the sound.

LOTS of electric guitar players guilty of the same.