what kind of controller are you using? i think with keyboard, its just learning the intricacies of your keyboard and how it responds. a ton of practice will allow you to get down the usmash. i know i definitely had a lot of trouble and i had the same problem where i would jump and do uair instead of utilt or usmash. on the keyboard, a lot of it is timing, same goes for the controller. get to learn it a lot using training mode, because even though i use n64 controller, it definitely reacts diff on my laptop than on the console, so i have to spend a lotta time just getting used to the d*amn thing.