This question came up on a daytime TV show last week - it must be in the news for some reason.
Personally, I think it is fine if the mum is dead, or entirely off the scene, but if not, no.
I would have resisted at all costs if my children had suggested calling someone else ‘mum’. I would probably have rationalised it and come up with some reason (it’s confusing, or whatever), but the truth would be that they are my children - only they have the right to call me ‘mum’, and I am the only person who should be called it. Jealousy? Maybe, but that is how I would have felt.