I don't think it's a Mumsnet phrase, it's been around for years. I've heard people who are definitely not MNers using it in real life. I don't use it but I don't mind it.
I am quite fond of "it is what it is" and "well, it is and it isn't" but only because my DH cannot stand either of them and so when he's annoying me I'll say them to him. I never use those phrases otherwise.
@PolkadotsAndMoonbeams - talking ten to the dozen means talking really quickly.