As early as the first pitch for the show, the Lick was supposed to be scary, and not funny in the slightest like all the other villians.
"He was first alluded to in the early series pitch, and was the only other character described in those documents in addition to the main six: Finn, Jake, Princess Bubblegum, Lady Rainicorn, Ice King, and Marceline. The pitch document states in all caps, "THE LICH KING IS NOT FUNNY," and states that the Lich had been imprisoned within his castle (although this was later contradicted in "Mortal Folly"). According to the pitch document, he uses a legion of undead skeletal knights to aid in his battles."
Give it time. I'm sure this will turn around to bite them all in the ass in the most serious of ways. The Lich is NOT funny. The Lich is forever. The Lich is the sign of the final end.
Fun fact: The inspiration to the Lich was this guy here, The Horned King, from Disney's The Black Cauldron. He has been the inspiration to several other villains as well, but the Lich from Adventure Time is the most notable one.