John was youthful when Mimi intervened in his life to take him away from Julia and John "Bobby" Dykins. When you're that young you assume your adult caretakers know what they're doing and are trustworthy.
In the case of Yoko, we have to remember that John's life had become (in Pete Shotton's words) a "continuous acid trip," and he had begun to surround himself with shady characters. Indeed, just hours before John summoned Yoko to his house and consummated their relationship, John had wanted to hold a press conference and announce to the world that he was Jesus Christ. He was not in his right mind.
Mimi, fortunately, wasn't able to stifle John's enthusiasm for his music, though it looks to me as if she certainly tried (see the other thread I just started on "The Two Women Who Raised John").
Yoko was more destructive in this regard, derailing John's life and career not once but twice.