I think it’s just a couple of silly subversions. A crustacean shouldn’t be in an ice cream shop to begin with, and when it orders “pond muck”, you’d think it’d just be a whacky ice cream flavor like a variation on mint ice cream, but it seems to be actual pond muck (something a crustacean would enjoy). Alternatively, it just found a new ice cream flavor that it likes, which is also silly.