It's a pun on Mona Lisa
Yes, I actually was going to answer that exact answer, but I read the thread again and I realized we're talking about a canonical reason why would Joe call his pizzeria Mona Pizza, considering Mona is just an employee and such. Maybe they're the only 2 employees and Mona helps with the management, so they thought of the name together? I could see Joe and Mona going around trying new businesses, kinda fits the crazy mood of the characters!It's a pun on Mona Lisa