"Mummy's boy" isn't about being close, it's about being controlled.
It's a negative because it means the son puts his mother first before the needs and wants of his partner and children. To word it another way, he puts the wants of his extended family before the needs of the family he created. It's not ok.
He does it for a 'quiet life' because he can more easily ignore his wife's needs than his mother's wants. This dynamic often comes with emotional abuse of some degree towards his wife, in order to stop her questioning the status quo or silence her if she does question it. That's shitty behaviour.
Behaviour he learned from his parents as to how to keep someone in line. Behaviour they deploy onto him to keep him in line. He in turn passes that toxicity onto his wife and children. And so the dysfunctional family dynamic continues down the generations, toxic parents creating toxic children who becomes toxic parents themselves and on and on and on. Until someone breaks the chain by refusing to comply with parental demands and refusing to have that toxicity in their own life.
Of course women are going to be closer to their own mothers. It's still usually the mother doing majority childcare in any family unit. And of course it's the mother who carries, births and often feeds the child. When that adult daughter gets pregnant herself, of course she turns to the person she's closest to! The person who brought her up and who's best placed and most trusted to help her with her fears and worries of pregnancy, to provide practical and emotional support after birth when she's feeling at her most vulnerable, and for help in raising her DC. The more useless her own partner is (and so many of them are) the more she's likely to turn to her mother.
What she isn't doing (in the vast majority of cases), is putting her mother's wishes before the wishes and wellbeing of her husband and children. That's why she's not "mummy's girl". As close as she is to her mother, she's putting her husband and children's needs and wants first. Not doing whatever is easiest for her and to hell with everyone else, like these men are.