I believe it started as a response to the "thigh gap" trend and grew from there. As in, if you don't have a thigh gap, that's okay, neither do mermaids. By fatlogic that translates to "mermaids are the spirit animal of fat bitches". Although, the also seem fond of Ursula from the little mermaid so it might just be that, as others have said, whales long to return to the ocean.