Pink Floyd - THe Final Cut
Happy Mondays - Yes Please
Primal Scream - Give Out But Don't Give Up
Donovan - Open Road commercially, Cosmic Wheels artistically
MC5 - Back in the USA (this was the first after Kick Out The Jams, right?)
Sigur Ros - Takk...
too lazy to think of more besides the obvious (Stones - Goats Head Soup, though I like it; Yellow Submarine: Bob Dylan's Saved and Self-Portrait, etc...). It's hard separating "disappointing albums after their peak" and "disappointing albums after a good one".