Perfect Blue - What was *that* scene?
This is more-so speculation on my part, so sorry if that this answer doesn't suite well enough.

It is not true.  Mima did not actually perform and the manager was not performing as her.  The image that we see is part of Mima's delusion as she originally doesn't know that her manager is pretending to be her, and truly believes that the person behind the blog is some sort of demented version of her.

Because Mima is unable to separate herself from the blog, she sort of enters a state of psychosis and through that altered reality, she sees herself performing.

The crowd most likely was cheering on the group because it is stated earlier in the film that the duet the remaining two members did had hit the top charts.  So it is probable that the fans are celebrating the group regardless of Mima being there.