That too. Everyone else on the crew has their own specific dreams and goals to work towards. I guess Jimbei could work to forward human-fishman relations, but it doesn't seem like he could do that on the Straw Hat crew.
Or maybe Jimbei would like himself some nice revenge on the bastard who killed WB?

That aside, then i personaly think he would be a excellent fit for crew, the powerup that the strawhats got should more or less place him so close to the rest of the monster trio that i wouldnt think the outcome of any machout were desidet beforehand.

As for group dynamic, then i do think his serious nature would make him an excellent straight man to the more crazy members of the crew.