Every time I make fried onions, they come out mushy and not crispy at all, anyone knows why?

Could the oil both be hot enough... should be 350-375 degrees... and the onions should by dry.

There is an article of how to get crispy fried shallots without a batter: https://food52.com/blog/15587-how-to-make-perfectly-crispy-fried-shallots-without-flour

I think this would work with onions too. To the question of why yours may have been mushy, I can only assume that you didn't cook them long enough to drive off all water or drain them from the oil.