Is there any good reason to keep Baby Ariel in the game?

If you don't like Sims who are immortal and will forever be teenagers as your generations grow up, potentially talk to her and die before a single day passes for her, I would say no.

Otherwise it also depends, do you like having real world people in your game? If also no, deleting her works too!

I don't even know who she is ontop of only knowing she's real, so it's one of those "influencer" cases which helps on the deletion side