I've been looking around for weeks on a solution to this issue. I've tried an XBox One Controller, a Pro Switch Controller, a Ps4 Controller, and a Gamecube controller. No matter what controller I use, I have the EXACT same issue. The main stick in particular (an on some controllers, to a lesser extent, the c-stick) is horribly inaccurate in-game. I swear to god I'll have the stick as perfectly horizontal as possible, and it'll somehow treat my input as a vertical input. This results in the game being rather unplayable, as I constantly get recoveries, tilts, dash attacks, and smash attacks all misinputted. It seemingly happens at random, but I can never go more than a few minutes without it occurring. I have tried changing the radius and the deadzone but sometimes it will either seem to reset, or the issue will still occur regardless. Also, I can not simply use the keyboard, and I'd rather not go into too much detail why. But using keyboard is out of the option for me personally. Please, I just want to be able to properly play the game without having to worry about being screwed over by a misinput.

Increase the dead zone

The dead zone is too low.
The dead zone is the area where the input is read and transmitted. When the dead zone is really low, initial slight movements will lead the controller to think that you're tilting in one direction, when the stick is tilted in a completely different direction.
Now I don't use a controller, so I can't suggest a setting for you, but try raising the dead zone and see what happens.

I'm not trying to come off as rude, but I already tried changing both the deadzone and radius.

Sorry then. Guess you're screwed. Use keyboard like how the game intended you too lol

Did you calibrate your sticks in the Control Panel?

If the answers is 'yes,' then I don't know what to tell you. The Direct Input feels very early tested/incomplete at the moment, which is a real shame. I don't use it myself, nor could I honorable recommand anyone else to use it. You would get more responsive inputs from a Keyboard > Controller program (Xpadder, Anti-Micro, Joy2Key, etc.)

Boy, if you don't get-