New head canon.
Marceline loves to play with hair. Normally she plays with her own but when she shaves it ( or shape-shifts it ), she goes and finds Bubblegum. And Bubblegum used to make her stop and tell her to leave, but after a while it just got too tedious. So she lets Marceline mess with it. The only thing she doesn’t really like is when Marceline braids their hair together ( because it’s incredibly hard to get undone. She is made of gum after all. )
On another note: CAN’T STOP WON’T STOP THE BUBBLINE.