Depends on what type of startup you are in and what college pedigree you have.
PM is a job, where you learn many things on the job. If you feel you are learning a lot already, there's no point in doing an MBA.
You already have real world experience of working in a startup. MBA to me is just a really expensive networking program. You don't learn to build business or anything about product management there, you learn how to manage a lot of things and network with people.
But still if you wanna go to IIM for the brand and tag it brings, there's nothing wrong with it.
But have a clear plan about what do you wanna achieve out of an MBA.