What exactly did Morrison change about the Joker? He had him split his tongue (making speech impossible, but that's bye the bye) and scar his face (like it needed scarring), but as far as I can tell his big Joker story involved the Joker undergoing some sort of psychological metamorphosis and emerging as... a twisted homicidal maniac... which he had been all along anyway.
The Joker in RIP was no more twisted or evil than the Joker who tied Batman and Robin to giant birthday candles to celebrate his birthday back in the 1980's except he was, if anything, less creative and more pretentious about it.
I like Morrison's work in general, but I couldn't in good conscience say his work with the Joker was inspired or even interesting. The complete lack of any other writer using the same Joker as Morrison has says quite a lot in that regard too.