She could just be an abnormally small whale, remmeber that every once in a while when parents give birth their infant can grow up to have some mutations. Like a third claw or a mis-structured hand.
And well of course she is a teeanger whale, and technically you are not a adult until you are 18 years old, so Pearl could also just be having some delayed growth spurt effects as well. I think The Chaperone was the best Spongebob and Pearl episode, but imho I don't really care for Pearl that much, I kinda like and think she's alright but she is actually one of my most least favorite characters on the show.