How to Fight
After grabbing all of the items around the room, including the Saltwater Gun, and healing in the Hot Spring, go fight her. She is an incredibly hard boss that'll kick your butt if you are not ready. The only real way to win this is by spamming PSI and Items. Of course, Bombs aren't very effective, so this is the only good time to truly use the Saltwater Gun, due to the area boss being sort of easy. The items she drops are much better than most of the items obtained in this game.
Mrs. Marshmallow only appears in Thunder Tower in Mother 3. Once the tower blows up, she will never be seen again. She might show up in the Final Credits though.