Because, if you think about it, he has the ultimate weapon... Toodles. Every episode, Toodles gives Mickey 3 or 4 items that he needs to complete his goal

So, technically, if Mickey Mouse made it his goal to defeat Goku, Toodles would give him the things to do so

Now, if you're on your way to ask what exactly he'd give him, imma just say, I don't know. Only Toodles would know -_-