In Real Life, mental illness of any kind is rarely a laughing matter. In media, however, it can be played with for humor. When a character swings wildly back and forth between different versions of himself, it's almost always used this way.
It's also far too often referred to as "schizophrenia"; while the word literally means "split mind", Psychology Marches On and nowadays schizophrenia is a [...] [tvtropes.org]