areivim, it would obviously depend on the person. For some people (me included), they cannot watch people dancing off beat. Others (possibly those that dance off beat themselves) probably wouldn’t care less whether or not the bride/groom are dancing on beat. If the music is geshmak, I’d continue dancing and let the star of the night enjoy the attention. Otherwise, I’d stand on the side and watch!
Interestingly, many times at weddings when I go dance in the middle with the kallah, they say something like “uuuh, finally a good dancer”. Maybe they’re feeling self conscious maybe they’re happy someone can help them dance on beat. Wouldn’t know.