Sans: Why did the skeleton go to the party?
Papyrus: Why?
Sans: 'Cause he was too fat and ugly!
Papyrus: AHHAHAHAHAHAHAHAHAHAAHHA LOLOL,OLOLOL
Alright, listen up, smarty-pants (or not-so-smarty-pants, considering you need me, Explain Bear, to break this down). The joke is that the skeleton is fat and ugly, which is kinda funny because skeletons don't have fat or skin, they just have bones. It's unexpected and silly! Skeletons are supposed to be scary, but this one is fat and ugly which is an insult!