From the Super Mario Wiki
Queen Mushroomkhamen appears to be the guardian of her tomb, where she and her son Prince Mushroomkhamen rest in mummy cases. Apparently, she will awaken if anyone enters the tomb, and will pursue any potential thieves.
When Hip and Hop Koopa heisted Prince Mushroomkhamen's mummy case from the tomb, Queen Mushroomkhamen awoke and chased after them, although she was unsuccessful in retrieving the case from them. Outraged at the loss of her son, Queen Mushroomkhamen went on a rampage through a local bazaar, scaring everyone out of the place.
When Mario, Luigi, Princess Toadstool and Toad arrived to stop Queen Mushroomkhamen, she wound up mistaking Mario for her son, due to the fact that Mario bears a huge resemblance to him. Mario tried to get away from Queen Mushroomkhamen, but she ended up catching him, and brought him back to her tomb.
Queen Mushroomkhamen refused to listen to Mario babble on about how he wasn't her son, wrapped his body in canvas, and forced him to take a nap in her mummy case. Eventually, Queen Mushroomkhamen fell asleep, giving Mario a chance to escape from the tomb. However, Mario's getaway awoke Queen Mushroomkhamen, who broke out of the wall of the pyramid to chase after him.
Mario was then saved by his friends, who had successfully retrieved Prince Mushroomkhamen's mummy case from Castle Koopa. With her son back, Queen Mushroomkhamen scolded Mario, but was quickly sidetracked by Luigi, whom she mistook for her husband. Not wanting to suffer the same fate as Mario, Luigi ran off, while Queen Mushroomkhamen chased after him.
Eventually, Queen Mushroomkhamen learned that Luigi was not actually her husband, and presumably returned to her tomb to continue her eternal life of sleeping.