Well, MMA fighters train in various disciplines, including striking. Boxing is not as well rounded as, say, Muay Thai, but it's still excellent and many fighters choose to go with it. Ali was strong and agile/fast for his size, he'd do well in MMA, just obviously lacks versatily in the end, in his weight class he'd likely lose to most (good) guys.
As for comparison of the two, Kimbo was honestly, not a very good fighter, just sluggish, too big and lacking in skill. No doubt Ali was superior, though due to Kimbo's ground game, superior to Ali's obviously it could be a decent fight.