I don't think that's necessarily true, although I agree it's a very common interpretation to make. Nibble said the words that he did. Nothing more, The meaning that others choose to make about the words is down to them. Whether the mum chose to interpret it as 'he thinks I'm a bad mum who can't...
I don't think it's rude, unless it's done in a rude way.
Fact is, the kid and his mum were in a public space and their behaviour have an impact on other people (not just to Nibble but other kids and parents around), so well done to Nibble for showing some courage and making a stand, and a...