As someone who was a fan of Peter and Mary Jane's marriage and who considers One More Day to be one of the worst editorial decisions ever made, I believe Dan Slott is making a very valid point. The reason why Mary Jane became such a popular character in the first place was because she was such a vivacious, happy-go-lucky character who was not only the life of the party but someone who was fun to hang out with. Not to mention her seemingly ditzy demeanor made for some nice comic relief. What's more,
as Gerry Conway would state in an interview with Word Balloon, she was one of those rare love interests who didn't need the hero; rather, she was a free-spirit who could've had any guy she wanted, but choose him regardless. Yet after Peter and MJ did get married, admittedly much of her personality was indeed lost, and she was portrayed--often times in various types of pajamas, lingerie and 80s hair--as either "Mrs. Spider-Man," or the stereotypical worried and nagging housewife. And I agree wholeheartedly that whole smoking subplot was ridiculous, especially since it dragged on for way too long.
Though I will disagree with Dan on two points: first, I don't consider the loss of MJ's personality to be a product of the marriage. That's was more of a product of lack of quality writing from the 1990s. Because let's be honest--with a few exceptions like the Death of Superman, Knightfall, Age of Apocalypse, Marvel Knights: Daredevil and the like, superhero comics in the 1990s were god-awful, especially when it came to writing romantic relationships between characters. Yet there have been writers on Amazing Spider-Man who did portray Peter and MJ's marriage really well, especially J. Michael Stracynski (though he's more 2000s), Peter David, and J.M. DeMatteis.
The second point I would disagree with is that the "tortured back story" (which was pretty melodramatic, I admit) wasn't what diminished her being a happy-go-lucky character. After all, having a psychological explanation behind a character's behavior and personality doesn't automatically mean they no longer have that behavior and personality, or that behavior or personality somehow makes them less appealing of a character, do they? For example, we know why Spidey cracks jokes whenever he fights the bad guys--it's because he does it to throw them off their game, put whatever innocent bystanders are around at ease, and giving himself a boost of confidence so as to not be scared. And we also learn that much of his sense of humor came from Uncle Ben, and that by being Spidey, Peter is essentially channeling and imitating him. Yet with that explanation for why he cracks jokes, do we think Spidey's no longer a wisecracking smart aleck? Of course not. Same thing goes for Doc Ock having an abusive dad, an overbearing and manipulative mom who smoothed him and ruined his chance to marry the woman he loved, and being bullied as a kid because he was a fat bookworm. So Mary Jane having grown up with an alcoholic dad, a sick mom, and a sister who was raising two kids by herself, that she exaggerated her party girl demeanor in order not to have the face her own past, I would think deepens her as a character and doesn't negate the personality which made her a fan favorite to begin with. But I can see where those who would disagree are coming from.