I think Celestia's issue was she was trying too hard. She wasn't allowing it to be natural. I know the fans are stuck in the mentality that she has to be an amazing actress because she's a figurehead and they're always faking it... but the thing it, Celestia doesn't. She's quite genuine. A Royal Problem just established that she's exhausted, and hides it. But that's besides the point - actual acting in a play is something she always wanted to do, and her behavior indicates that she was truly trying too hard. When she pretends and acts all playful, that sounds real because Celestia's heart is in it.
They're completely different. A lot of us are the same way - we can joke and pretend to act serious, but when we have to act in a precise way, it's actually really hard. It's more than just saying a line a certain way.
What hurt Celestia, though, wasn't the fact that Twilight didn't tell her the truth of her acting. It's that she was hurt because she finally thought Twilight grew out of her awe and fear of Celestia, that she actually knew the real pony beneath the princess title; that she knew Celly WOULDN'T have been mad if Twilight told the truth. It's actually quite an amazing character moment for the two, because I've always wanted something like this. And I empathized with Twilight - she really just didn't want to crush her mentor's dream. She wanted to give something back for once.
