From Monkey Island wiki
Mabel is a dental hygenist who visits the Caribbean as a tourist.
Mabel can be found in StarBuccaneers on Jambalaya Island. She is a big fan of the chain and its coffee, she also loves shopping for exotic souveniers to take home and make her friends jealous. She admits that there is a StarBuccaneers near her home, but appreciates the far-off store more.
She has a mug in her bag that Guybrush can take to get free refills of iced grogacchino.