If it's SaaS, you should be monetizing from day one. An MVP in SaaS is the minimal product that people will pay for, that's the starting point. You evolved the product from there.

If it's a consumer app, that's a little more complicated. Often times, monetizing preemptively can stunt growth and kill the trajectory, due to lack of focus, user experience, etc. It's really a question of access to capital in this scenario.