The dangerous part of it is when people start to think that you are what you joke about and when you start to sort-of-believe your own jokes. You may know you don't mean it, but they dont and may end up believing it anyway. And if you repeat variations of "I am stupid" often enough, your confidence may go down. Is likely, even.

At some point, is it not humility nor positive anymore, it is just negative.

Yes, people like it when you insult yourself. Still, if the relationship requires you to take down and insult yourself regularly, it is not worth it. I mean it.

I had that verbal tick where you automatically take down yourself or underplay own achievement and in retrospective, it costed me a lot. It took a lot of effort to get rid of it and I am still not fully there yet. People like it, they laugh and are friendlier and nicer to you. But they do end up believing you are less capable, then you are, respect you less, your word matters less in things that matter.

Dont do it as a habit.