This would be an interesting case in trademark law (I'm no expert, .just studied it a tiny bit a while back).
"Passing off" ie pretending to be another company is illegal.
The mumsnet logo is of course taken from Charlie's Angels, but mumsnet aren't trying to pass themselves off as actually associated with Charlie's Angles in any way, and no one would reasonably think that.
But the cleaning company are trading on being mums, and they've chosen a similar blue to the mumsnet logo,.so they're on much more dodgy ground than mumsnet and Charlie's Angels. Could there be benefits for the cleaning company if people thought they were endorsed by, or something to do with mumsnet? Quite possibly.
Could they be done for passing off?
If mumsnet have traded their logo quite possibly. They would of course argue it's from Charlie's Angels but Mumsnet might well be able to argue they are trying to benefit from an implied association with MN.