OP, well done on getting your home and working so hard to make it how you would like it to be.
Congratulations on your happy relationship.
As others have said, I think your Mum is jealous, has not got over the fact that you moved out, and feels abandoned after the death of your dad and then you (quite reasonably) moving out.
She just can't bring herself to be happy for you.
This is her problem, not yours.
If there were signs of this before you moved out, and she has always been like this, then yes, she probably is Narc.
Either way you need to protect yourself from it and you certainly need to make sure your partner does not get caught up on it and have to put up with your mother interfering in your now joint home.
So, if possible a calm talk with your Mum along the lines of 'we seem to be getting upset with each other over the house..' and tell her you have appreciated her help but you need to make your own decisions now, for better or worse. That she has her home, and you yours. You hope you will each be welcome in each, but do not expect to be involved.
Invite her less often and for very specific things 'Would you like to come for Sunday lunch from 12-3 ?' And don't show her or discuss your home improvements.
Visit her to cut her off at the pass visiting you!
Practice a series of phrases to shut down her comments. "Each to their own...oh, is it raining again?" "Oh well, we love it, nowt so queer as folk...did you get your stuff back from the cleaners?" "LOL Mum, you always say that...never mind, as long as we are happy that's all that matters to Dp and me...did you see Wisting?"
Imagine her comments dropping off you like water off a duck's back. Actually visualise that.