From Monkey Island wiki
According to the waitress at Planet Threepwood on Jambalaya Island, Bertha is the name of Guybrush Threepwood's mother. However, she also claims that Guybrush is 6'4", has a big scar above his left eye and owns a big black parrot, so her information is untrustworthy. Guybrush himself never mentions his parents in Escape from Monkey Island.
Guybrush's mother, who appears in Monkey Island 2: LeChuck's Revenge and its Special Edition, has blonde hair and a big blue dress. She appears to be more talkative than Guybrush's father. Also, in the Special Edition she is voiced by Sally Clawson (who also voiced Captain Kate Capsize).