I honestly didn't mind her going Mad Queen as the last episode did set up for it, but I do have to say they didn't really stick the landing with the twist. They didn't really give her a good reason to finally snap and go off the deep end since as you said, they didn't really make it clear why it happened and at that moment everything was going her way. She wiped out the Iron Fleet and got revenge for Rhaegal, her soldiers suffered few casualties if even that, and King's Landing had completely surrendered to her, so nothing justified the final push into madness. It felt rushed to say the least, and they should've either given more attention to her deteriorating mental state or give her something to get angry over like having Grey Worm get attacked and/or killed in the skirmish to make it more fitting.
I also agree that Cersei's death was disappointing. I have heard some counterpoints in that it played into her relationship with Jamie, but all in all I feel the biggest thing going against the moment is that it just doesn't feel satisfying. Something was truly missing to sell this final big moment for the character and the way she was left off doesn't really give her a proper conclusion, which hurts even more since she is one of the main villains of the show and the only one that has been around from the start. I also didn't like how Jamie was handled, particularly with his relationship with Brianne as well as his battle with Euron not having a proper effect despite the latter stating he gave him a fatal wound.
Cleganebowl was well done though, it was easily the highlight of the episode and it felt fitting of both brothers' arcs.