I say, diss the other babies and add Li'l Yoshi!
I'll jump to that. Never liked any of the other babies.
But on second thought, it was good having Baby Mario in "Mario Tennis" just so I could beat him. Those annoying sounds just made the final victory that more victorious. I remember one time there was a heated battle, I accidentally slammed the ball right towards Baby Mario, very tense because I knew he'd slam it out of my reach, he's powering up to hit it... and the ball leaps over his head. With a "Waah!" the point was over, and I was so happy. I was also amazed that the computer was allowed to make a mistake like that. Goes a long way towards making the computer more fun to play against. Well, that and it's always hilarious to hear Mario go "dooooh!" when he misses a shot.
If Baby Yoshi were in Mario Tennis... well, that'd be weird certainly. I'd gladly switch Yoshi's current voice (the "Waaah!" "Oh-wo-wo-wo!" and other embarrassing sounds) so it'd be Baby Yoshi's voice, and bring the Yoshi sounds of the Mario Kart 64 days back to Yoshi. I bet Baby Yoshi would still be a thorn in my side after a while, question is would he be as satisfying to beat the snot out of as Baby Mario... I'd say no. So he's less annoying, but also less satisfying to beat. Decisions decisions...