Is it ok to cut an access panel into the drywall above my service panel?
Code requires the wires to be covered it doesn’t specify anything other than 1/2” Sheetrock or plywood. I do create access points in my home and some jobs that need them the only difference is I have a piece of trim over the cut usually held in place by Velcro. I can see an inspector having an issue with the cut especially if the wall is common with a living space , if an outside wall I doubt they would say anything.

Added for more info:
Note sometimes these flaps are cut so the clamps can be installed. I use a plastic clamp called “insiders” Raco brand, punch the knockout feed the wire through the hole then slide the insider down the wire into the hole and it clamps the wire. I used to have to cut those flaps but with the insiders I just have to fish the wire through the hole. 
Note I have no links to these other than I love them and have been using them for quite a while saving time and $ even on new installs.