Well logically Dimentio would being a future fame thee Geno so he'll naturally be strong but e we be piratical...Dimentio has lot of way to avoid Geno and well hate to say it but Geno while strong is sort on of the weaker characters, His magic does seem that strong and as poor defenses. Now that doesn't mean he can't. Dimentio is a trickster but he not really that strong himself when when using someone else does he gains advantages in that ares. I'll say it'll be a pretty even fight, not sure how Geno Whirl would work in a more action setting. If we talking RPG then I'm not sure as Dimentio was not design for that so he may have a completely different style.
Game mechanics mean nothing in terms of power. We have to go by story feats. Dimentio did more on his own and almost destroyed the universe. Geno needed lots of help to accomplish his goals. I gotta give this one to Dimentio. The main villains are usually stronger than any one party member.