To think MIL shouldn't have said this

What?
It's fairly straightforward.
We don't deal with male entitlement by telling women to behave in a certain way.

I don’t know what that means.

We don't with it by telling our girls and women to dilute themselves. 🙄

Of course it is, nobody with half a brain would disagree with you. But that’s more likely to happen in some clothes than others. We don’t live in an ideal world where all men are sensitive and respectful, we live in a world where lots of young guys are influenced by Andrew Tate and his ilk. We have to deal with the world as it is, not how we’d like it to be.

The actual problem is the unwanted male attention though.

Nice victim shaming/blaming.
Girls and women expressing themselves through clothing doesn't give anyone the right to act in a disrespectful manner toward them.

It's not like that at all though.
Stop victim shaming.

I am on your side OP. It was I inappropriate of MIL to bring this up and I hate people trying to police what women wear like that.

I got catcalled and harassed as a teenager wearing my dowdy school uniform. It really doesn’t matter what you wear.

What wonen should be able to do and what they can do are not the same thing . Because men are arse holes. That's reality no matter how much you want it to be different.

Sorry but with my daughter's I wouldn't have a problem with that being said. It's naive to think this world is all roses these days so would always make sure there dressed appropriately for there ages.

Some of the victim blaming responses here are crazy. My friends and I would often be catcalled by middle aged men when walking home from school in school uniform even as tiny year 7s. We certainly didn’t have our “tits and arses hanging out”. In this situation, i’d simply have a chat with DD and explain that she is not responsible for mens actions.

Really? You don't think that those images of young women we see with their tits and arses hanging out on High Streets on Saturday nights are not giving out a dubious impression ? Yes it may well be female empowerment or whatever other random reasons are chosen but the fact remains they are attracting attention to themselves. They are going to get the catcalls, the comments and the looks. You have to take some small iota of responsibility with this.

I don't think that grandparents need to be rebuked for every mistake they make, any more than grandparents should rebuke their adult kids for every mistake they make.

In this case I think the priority should be to let DD know your own feelings in order to undo any discomfort she felt, and to simply explain to her that your MIL is out of date and that things have changed a lot (re blaming women for what they wear) since she was young.

No she isn’t. She’s recognising that some forms of dress are more likely to attract unwanted male attention than others. It’s undeniable regardless of the feminist utopia some of you think we live in.

There is a joy sucker type that love to rain on a happy moment.

Dismiss her for old sourpuss she is.
Just tell your daughter how fab she looks.

Its a balancing act for mothers to celebrate our daughters AND try to keep them safe.

She is making a 13 year old accountable for the behaviour of adult men. I would absolutely have interjected in the moment.

Yikes

I do think the MIL was wrong. There's a time and a place for everything.

That 13 year old girl, instead of looking at her new dress and thinking it looks great and she's pleased to wear it now has had a suggestion that implies (at the back of her mind) the idea

that there was something wrong with her choice of dress,
that its too provocative,
that she will be dressing in a way to attract unwanted male attention
that she herself has chosen something that will generate this attention so it will be her fault.

That's very undermining and could damage her confidence in herself, make her worry about what she's wearing. Especially as she was so delighted with her choice originally. She will doubt her judgement after that correction.
Why couldn't the MIL let her 13 year old Granddaughter just enjoy showing off her dress without being left with the impression that there's something wrong with it?

Of course young girls/women should be warned about personal safety... but not like that.. I used to feel that way about relatives making comments about children's weight/eating habits/diets.... They are sensitive and someone in their family circle making stupid comments about these kinds of issues can leave a very poor impact on them.

Ridiculous and regressive nonsense.

Actually, I am not wrong.

I never said it doesn't happen regardless, I said that it's worst when you wear certain types of clothes.

If you are a woman, surely you must have noticed it by yourself.

well, it would have been better to say something there and then. No need to make a scene, but sulking and asking your husband to have a word is not a great example for your child.

MIL “needs telling” what exactly?!

To mind her own business and let the child's parents decide what is and is not appropriate, with regards to their own daughter.

I don't think there's any need for either of you to be 'having words' with MIL. She wasn't implying blame, and she didn't intend anything other than protectiveness.

You might prefer she hadn't said it (as would I) but I don't think she deserves a telling off. From your DD's perspective, your life attitudes are more important than something that a grandma said. So you just confirm that no woman or girl is to blame for men's actions, and talk about her safety in general terms. This one off comment from someone two generations above your DD will not have the impact that you seem to think.

I can't help feeling that there's the fact that it was your MIL saying this that was a factor.