Mermaid girl with an incredibly chunky tail. She constantly has to rest on rocks after swimming for too long, and as the sailors see her, much like her sisters, she tries to lure them in, her belly hanging from the rock and touching the topmost part of the water. However, the sailors scream "whale!" Or "manatee!" As they see her, making her pout in annoyance and drop back to the ocean making a huge splash that makes the sailors wet, as she goes back to her kingdom to eat some caloric food
This is wonderful and very much love the idea that merfolk would be fat for the sole purpose of their (semi) mammalian biology being the cause of them requiring blubber, but if I may add onto this...
A merlady who’s been relegated to working at an amusement park or aquarium, where she “works” by just hanging around in a spacey tank, greeting guests and generally being a friendly, lovely person: to the point where innocent people and children throw her a bunch of snacks and food for her just, well, being there. She keeps on snacking on the human food, even going as far as to asking the staff to get her some after hours, and slowly fattens up nicely, still surprisingly athletic and acrobatic but now: ADORABLE ‘n PLUSH! Like, hugging her would get you wet, but she’s fat as hell and feels like a sea-pillow.
Or... plump pirate merfolk, only raiding ships for the sheer purpose of stealing their food and getting fat off of it.