It's pretty simple to see that Ness would win.

Mog may be cool, but when it comes to a battle, what can he depend on?

Mog would probably try sending a few weak spells at Ness, only for them to be absorbed. Then, he might try attacking Ness with his puny little sword. Like that will do much...

Ness would nail Mog with his yoyo the second he got close enough, and if that wouldn't be enough, Mog would be pwned with his home run hitting baseball bat.

Batte = Done.