Mukuro was interesting before because she was more grey. She was mentally abused by Junko, but totally devoted and felt guilty for leaving her behind for Fenrir. DR1 chapters 1, 6, FTEs and School Mode made her seem like a more interesting and conflicted person who, while evil, wasn't so much in the same way as her deranged sister. Also, her being more mysterious helped attract people to the character, and people generally felt bad for her after her sister's betrayal.
Her final scene in Despair took the air out of that though, as she was probably closer to being happy to die for her sister. Like everything in Despair, she ended up being hamfisted fanservice. Which is a shame, because they could've explored her in a more interesting way than just a lovesick puppy. Exploring how Junko had her under her spell or even displaying it more would've been something since they didn't give her much interaction with anyone else.