I think that the people whom have given this movie a bad rating have actually forgotten the first movie. I feel this holds very true to that origin. It is not really a musical. Yes, it has some music. Yes it has some dancing (and I was surprised by his singing voice, too) but, if you've actually paid attention, so did the first movie. Do people really not remember him dancing on the stairs or in the bathroom or in the apartment in the first movie?
As for the delusional scenes- that was in the first movie as well. He had an entire delusional relationship in the first movie. So don't avoid this movie because of the reviews; instead, keep it in context.
I think the real reason it got a bad rating from joker fans is because they wanted to see a supervillain blossom.