Too Much Jelly (Patreon)

You see, the title is actually a very clever reference to the 1823 poem "A Visit from St. Nicholas" (also known as "The Night Before Christmas"), wherein the titular character's large belly "shook when he laughed, like a bowl full of jelly" - in an example of both metaphor and rhyme, demonstrating Clement Clarke Moore's literary skill.

Here, too, Melony (The Circhester City Gym Leader from the game Pokémon Shield, one of the games that kicked off Pokemon's 8th generation when released in 2019 for the portable hybrid Nintendo Switch console) has a sizeable gut, and is attempting (and failing) to dress in a Santa-like manner, establishing a clear thematic parallel.

And thus, I have explained my joke. You may now laugh. Ho ho ho!