From Muppet Wiki

Mrs. Bluveridge is the proprietor of the Admiral Benbow Inn in Muppet Treasure Island. She's a tough and rugged lady, and holds her own with the shady characters who patronize her establishment.

Her character was created especially for the film, and did not exist in Robert Louis Stevenson's original novel Treasure Island. In the novel, the protagonist Jim Hawkins is being raised by his mother; in the Muppet film adaptation Jim is an orphan and Mrs. Bluveridge is the closest thing he has to a guardian.