It's hard to deny this claim. Most people who knew the Ono-Lennons said that Yoko was the more dominant partner. John himself said of his beloved wife, "Yoko looks upon men as assistants... Of varying degrees of intimacy, but basically assistants." Many people, including Paul McCartney, Cynthia Lennon, and Yoko herself, have made the connection between John's Aunt Mimi and Yoko herself.
And yet there's another side of John that few people deem worthy of comment: He liked to dominate women, too. Stories of his relationships with non-Yoko women, especially Cynthia and May Pang, demonstrate that John wanted to dominate in those relationship. John is criticized for his abuse of those women, but rarely is the fact that he wanted to dominate them considered worthy of mention on its own.
This is despite the fact that these make up all of John's romantic relationships with women, excepting only Yoko. And even with Yoko it was more complicated: According to Yoko, John would tell she was like a "bloke", like a "man in drag". In order to accept domination from a woman, John had to think of her as being in some ways like a man.
So why is "John wanted to be dominated by a strong woman" repeated as a truism without pointing at that John more often liked to dominate? My best guess is because it's weird. Men wanting to dominate women is normal. Men wanting to be dominated by women, especially men like John and women like Yoko, is weird - at best. Often, the response is outright disgust. Over the years, most of this disgust has been directed at Yoko, but John has received some, as well. He lacks balls, critics say. It's freakish and weird and worth commenting on.