Ground pour underneath an MCU
Ground pour on the top layer is not really doing much. Having a ground pour on the top would make make sense if you didnt have a ground plane on the bottom. So definitely add a ground plane on the bottom and try and keep all routing on the top.

What I would do is have a power pour on the top, and a solid ground plane on the bottom. Now all your return currents have somewhere to go won't fringe across gaps or over other traces.

The ground pour on the top layer (under the microcontroller) does nothing. It has no harm, but it really doesn't have a benefit either (assuming you have a ground plane).