How to fix armoire to bent wall and mask the gap behind it?
Your plan is fine (and exactly what I'd do). Most furniture has a small gap behind it anyway, so it shouldn't look bad.

The only other alternatives that won't look odd would be to actually shape the cabinet to fit the wall--the side panels and top would need to extend back at an angle. Trying to conceal the gap with trim boards might just draw attention to the matter. 

If you wanted to, you could wrap the entire back perimeter with coordinating trim in whatever manner suits the style of the piece.