From the Super Mario Wiki
During her story, Mona visits Joe's Clothes to find a dress to wear at a party she is attending later on. Joe sees her and confronts her, offering to help her find the perfect dress, which she describes as "extravagant", "cute", and "cool". This confuses Joe, but Mona is able to spot a dress she likes anyway.
Eventually, Mona manages to find the perfect dress, and invites Joe to her party, which she realizes that she is running late for. She then says goodbye to Joe and leaves in a hurry on her scooter.
Joe's Clothes is seen much later during Wario Deluxe's stage, along with various other locations.