Some facts about Sans:
He was forced to wear something more professional by Papyrus once he shared the news of his newest employment since he was going to be Asgore's assistant.
The reason behind the change in jobs is because of the fact that a royal judge just isn't very high in demand in the underground. The local authorities handle most cases fine and monsters are pretty peaceful.
And since it's been several decades since the last human sighting (very few monsters alive in the underground know how a human looks like), it was high time for a better gig.
My headcanon is that Asgore is dyslexic (Dreemurr being a misspelling of the word "Dreamer" being the primary reason) and Comic Sans is famously dyslexic friendly. Hence why he was hired and not somebody else.
(Explanations for my thought process below:)









