To be honest i do think an angelic King Boo would disturbingly fitting
It was shown that since the first LM, King boo became progressively more and more insane, an angelic King Boo would be disturbing because it would symbolize that he has lost pretty much what was left of his sanity since the events of Dark Moon.
Because he would act like a happy angel which cause paranormal chaos, he hardly think rationally anymore, so he doesn't even try to be scary like a boo, basically in DM King Boo look scary because he is enraged, in LM3 he look angelic because he is insane and completly forgot what he was.
And that fit for the master of illusions theme, he look angelic yet this version is possibly the most destructive of the series.