I'm planning to call my niece's pun stupid
Is there a problem with the pun?
No, but her mother called me over to hear it, and I wasn't impressed
You shouldn't call someone's joke stupid, especially if they are a child. If you thought the joke really wasn't that great, you should tell her mother in private, so that she won't over-hype it to others in the future.
Fine.
Good. Being so blunt to a child isn't a nice thing to do.