AIBU to be upset by my mum’s “jokes”?

My MIL did this when DS1 was a baby (first grandchil). For example, his socks might be twisted and she'd comment something like "what's mummy done to you now?" and sometimes even "is mummy being mean? What a bitch!" It drove me mad.

I decided eventually that she was struggling with no longer being the only mother in the room and wanted to assert her matriarch status.

Could this be the case with your mum?

That sounds really annoying, I'd ask her to stop for that alone!

Well, if she ignores you directly asking her to stop then I would just say 'silly granny has got in a muddle again! You bumped into my legs didn't you!' etc.

She'll soon stop...

Your mum sounds pretty dense, and fairly tiresome.

God help you if your child ever repeats what your mum says to nursery/anyone else. You need to stop her.

Put on a stern voice and correct her immediately every time she does it.

"Oh no did mummy push you over? I saw her! I saw her!"

"No, I did NOT. We do not joke like that in front of the kids. Do not put harmful ideas into his head."

Immediately, every time. Make sure your dh etc are on board because her instinct will be to try and laugh it off or make you out to be mean. Dh can just reiterate "we don't put harmful ideas into his head"

YANBU to be upset by her stupid remarks, but you also don't need to worry that it's going to damage how your DC thinks of you or any lasting issue there. Sounds to me like your mum is insecure now that you've grown up and become a mum, so she needs to undermine you and make herself feel more important. It's pretty sad, but she's the insecure one not you, so you can be quite calm about telling her the jokes aren't funny and to stop making them. And if she crosses the line when you've drawn it, keep her at more of a distance until she gets the message. Show her you won't be belittled like this.

Basically yes. Your mother is not a smart woman. You will need to explain to her in words of one syllable why she needs to STOP. IT. She won't fully understand, and there may be eye rolling, huffing and future instances of exaggerated caution "Ooh, must be careful! Silly Granny mustn't annoy Thunderbird7!" but she really needs to stop it.

Once or twice a blue moon, then yes funny. But not continually like this, it's really not on. I think you need to think about how to word it and have a chat with your mum about it. Like you say, you don't know how much he's taking on board and one innocent phrase as he learns to talk, could lead to a heap of unnecessary trouble!
Normally someone would comment "Whoopsie daisy" or some such like, if a toddler fell on their bum or did something clumsy? It seems an odd thing to me for her to comment like she does? I'm struggling with that one!

My mum does this too! I think she’s trying to bond and have an ‘in joke’ with her granddaughter through these comments, which is ridiculous. ‘mummy is annoying you isn’t she!’ ‘What’s mummy pulling your hair with that brush for? Yanking you, naughty mummy!’ I asked her why she does it, and she got VERY defensive saying ‘I’m only joking’ and huffed and puffed. I’ve told her to stop and if she does it, I say ‘silly grandma is joking, I would never do that on purpose.’ I feel for you!

If he repeats this and tells teachers at nursery (for example), they could get the complete wrong end of the stick.

Yanbu your mother is going to land you in all types of shit in future what if ds runs into nursery repeating her one day?!! Social services that’s what! I’d be having a firm word…...

I think you need to explain your concerns to her, and that it upsets you. I don't think you're unreasonable, but getting upset and not saying something is.

She is being a bit thick and needs to stop. What if he starts repeating to someone outside the home!

You're not being unreasonable at all. Those comments are not funny and are mocking actual child abuse and cruelty.