Don watching Megan’s screen test is a critical moment. Clearly something about it changes his mind and we see the season’s arc-of-change really come into focus after that. Yet most people seem unclear about what happened in that scene and it’s critically important to what comes next:
Don is watching a screen test of his wife. This is a reel of film designed to show an actor’s ability. She’s going through her various poses, looks, etc. Don recognizes those looks - they’re the ones he thought were just for him, the sideways glance, the eyes turned down, the hair moved slowly behind the ear.
Previous seasons have shown that Don really understands when he sees things on film. It’s a recurring theme going back to S1. After watching Megan’s screen test he cringes a little. He’s getting wise to her act. And he sees that it worked on him. It must be pretty good. So yes, she can be an actress, but maybe he doesn’t like actresses.