Never thought about this, actually. But now that I do, it's pretty baffling. There are no negative associations with being a "daddy's boy/girl" and even so, usually being a "mama's girl" isn't bad, either. But make it mother and son and it's the butt of a joke more or less.
I don't know, maybe it has something to do with the idea that a guy who's close with his mother isn't as independent as he could be. Considering mothers usually are the primary caretakers and the ones who (try to) answer to the child's every need. If that makes any sense, just a thought on the spot.
That being said, I might as well admit that I would most likely count as a mama's boy. My dad has never been around so mom was the only parent I had. I know it's not easy being a single parent but she did pretty damn well, considering she's been diagnosed with mid-level depression and she has a history of panic attacks as well. She's hit rock bottom several times during my life and dealt with terrible thing domestic violence but now she's better than ever, married and has a steady job that's perfect for her (she's a practical nurse, working at the local center for disabled people).
She's not only my mom, she's also the one I admire and trust most in this world. I'm proud to have her as my mother and could care less if that makes me a mama's boy.
...Sorry for that outburst. >.>