Kutcher is only funny as a dumb ass; a sort of verbal slap stick type of humor (if you like that sort of dumb ass humor).
That wasn’t the form of humor Sheen’s character did and I think Kutcher does not know how to do comedy, other than as a real dumb ass.
If his looks were less attractive, he’d be better playing the role now played by Sheen’s former co-star - the dumb ass brother role, than he will be in Sheen’s old role.
I don’t know any of his “comedy” but he wasn’t bad in The Butterfly Effect.”