Haha. What a twat your MIL is.
But seriously. She does not mean well, and it's so obvious that you really do need to sit your DP down and have a little chat.
And this is what the chat should be. Very nicely and with lots of smiling, tell your DP that firstly, if he wants you to respect him and think of him as a good, functional partner and adult, he needs to stop patronising you - as well as making himself look stupid - by defending his rude, childish mother in such a ridiculous way when her actions are so overt. It doesn't make you think oh well maybe he's right - it makes you think that he is either deluded, spineless and afraid of his mother, or (worst of all) ok with her snubbing you and is letting you know that when push comes to (ridiclous, petty, laughable) shove - he will defend her over you.
And that is important. A nasty snubbing present is one thing - she has shown that she's unpleasant, but she can be avoided, and you no longer wish to have a relationship with her anyway (more on that later). However, his response alarms you. If he's going to stick up for her when she attacks you then your relationship will not last. Bottom line. Smile smile. No, it can't be ignored. It's not about a present, it's about his loyalty to you, your relationship, and boundaries around how he's prepared to sit back and let people treat you badly.
Which side of the fence does he want to be on? Answer needed.
Secondly. After three years, there's something he needs to know. You don't need or want to have a relationship with someone who dislikes you, be it MIL or anyone else. She's got the right to dislike you, too - childish and nasty though it is when you've done nothing to upset her. However. He needs to know that NOBODY who treats you badly will be tolerated as part of your life and family - again, MIL or not. So, he needs to understand right now that for example, if she keeps up her behaviour, what IS going to happen is that should you two marry and start a family, MIL will not be welcome to be a part of it. No, don't say 'but she's my mother' - if she were a good mother and therefore a potentially good granny, none of this would be happening, because she would be supporting his choices in life/relationships, and not treating his partner like shit. So, just to let him know (big smile!) - if we ever have children your mother will be lucky to see them once in a blue moon, and more likely she won't have contact at all because I will not have a toxic witch filling my babies' head with negative shit about their mother. Just so you know. She WILL see the consequences of her behaviour.
So. To summarise. Stop with the shit about your mother meaning well. It's making me respect you less. We both know she hates me because she's a nasty twat, own it, it needs acknowledging or it will wreck our relationship. Secondly, know now that I'm fine with her hating me, up to her, but if she thinks she gets to turn round the second I give birth and be right in there with my baby she will feel my foot right up her nasty fucking arse. Ok?
Big, huge smile to finish!