Celestia’s pulled her posture back and up in her seat, stunned by the rebuke. Or stunned due to a more focused assault on Cadance’s part, but that’d achieve the same end anyway. Maybe just stunned because she dropped the whimsy for once. You would be stunned too, if you hadn’t half a mind to chew out Celestia yourself – more politely phrased, of course – if she’d gone on long enough before Cadance arrived.