It's a rare mum who doesn't have an opinion about a present or future consort of their son. Mum's standards are very high, and few women can reach that stratosphere. It's irrational, but it's your BF's mother
She's talking to you and is appearing to be decent about it. She had a bad day. You'll have a few too, with her, trust me. Put it in the past and move forward. Your BF's bond is moving from his mother to you. He's always going to be her "boy" no matter how old he is or how many grandchildren you provide. That's just the way it is.
If he's not an only child (I am), thank your deity of choice. Only sons are gods in their mother's eyes. I dealt with everything. Too bad my mom became demented before she could really love my wife. ...