Celestia is NOT bad at acting. She’s actually very good at acting. So good that in ‘Horse Play,’ she fooled everybody, including professional actors, into thinking she
couldn’t act so she could pull an ‘Emperor’s New Clothes’ maneuver and see who was honest enough to actually tell her she was “bad.” She wasn’t expecting much from most of her subjects (and was just toying with the Method Mares for her own amusement) because she was mainly trying to test the Mane Six, and especially Twilight. When she caught them talking about her behind her back, she feigned offense to shock Twilight into doing better when in reality she was just mildly disappointed.