AIBU to think my SIL was rude about my clothes?

have a girly day out shopping?

Or have root canal surgery, far more pleasant.

You don't need to be irritated, she's just trying to be nice and you should take it the same way. Why not let her take you shopping and push you out of your comfort zone? Bond over some nice shoes, etc
You should just accept everything life offers with joy instead of negativity. If you don't want to buy new stuff she can't make you, but go out anyway and try something new!

Maybe the OP could say 'That's very kind of you, let's take your credit cards for a day at the shops.'

When I was in my mid-twenties, a co-worker (same age) said "oh my mum has that blouse" followed by a smirk and side-eye to her friend. I felt so humiliated.

Well, I think you can. I think there’s a difference between looking down on someone (which is what she was doing) and wanting someone you care about to feel their best and take care of themselves.

For example, saying, “You’ve been through a lot lately. I’d love to take you shopping or something if you’d enjoy that. I think you deserve to do something nice for yourself,” comes from a place of care.

That’s very different from, “You always look drab. You need a makeover.” One is about supporting someone and helping them feel good about themselves, the other is just criticism.

I think SIL thinks you are very badly dressed and is embarrassed by you. It sounds as though she thinks you are stingy about spending money on clothes and are looking dated and dowdy.
Are you a bit depressed? Have you given up on caring how you look?
Your SIL is tired of seeing your hibiscus dress. She thinks you need a refresh, because you look stale.
( three quarter leggings? Really? I didn’t know they still existed. )

I don’t think it is ok to ‘want to spruce someone up’ who never asked. SiL needs to mind her own business.

What a rude cow. Denim shorts and black leggings are always around. They aren't high fashion but aren't "dated" either. She sounds a bit of a fashion victim if she thinks women should only wear the latest trends.

Linen trousers, yuck. You might as well tie two creased paper bags together and wear those on your legs. Same effect.

When I was a young teacher, I received compliments at work for a new outfit and was asked where I’d bought it.

”The M&S sale.”

My HoD, who wasn’t involved in the conversation: “Oh, M&S is all right if it’s just for work.”

It reminds me of a coworker I used to have. She looked at my dress and said, “That’s a good color for you.” I said, “Thanks.” She replied, “ I used to really like that style dress when it was in.”

All these people calling SIL names and that she's rude - it may not have been what you want to hear but the fact she didn't say these things in front of everyone and suggested a shopping trip together I think means she meant well - maybe next time you see her, take her to one side and thank her for her offer but explain you are happy as you are.
If after that she still continues, then yes she's being a bit of a b*tch but I've had in-laws say far worst things to me, so, unless you're feeling your look is perhaps dated and just needing validation that it's not, I wouldn't worry too much. My husband gets me tops for birthdays, etc, and often my first thought is 'that's not my style/I'd not pick that' but wear them to not be rude and then I find, actually, it works well - just gets me to step out of my comfort zone and try new things. He certainly doesn't do it with malice or because he thinks I don't look good or wants to control what I wear (before people start on that) - he simply sees something he thinks would look nice on me, and 9 times out of 10 he's right! I tell people he's my personal stylist - my problem is I can't picture myself in clothes/outfits without trying them on first - and most of the time I can't be bothered with that kind of fuss - so these 'surprise' outfits allow me to vary my wardrobe without the hassle. Maybe SIL is just trying to encourage a bit of diversity.. and those saying flared linen trousers are awful - I did too, until I tried a pair and then actually found them quite flattering, cooler in the heat and a bit smarter than my usual leggings.. The SIL could've been much ruder if she was so minded.

Over the half term week we saw dh family a few times. A kids birthday party, family bbq and couple of outings to the park and pub.

I admit I’m not very fashionable , don’t really follow it and wear what I like and what suits me. For context this is what I wore each time Kids party : black 3/4 length leggings and striped vest top and converse
family bbq hibiscus pattern dress and sandals
park denim shorts and a vest top and sandals
pub same hibiscus dress (it’s very old I’ve had it years but I love it)

The first time SIL said ‘oh here’s Y2K girl’ and laughed at the dress. The second time I wore it she quietly said to me ‘a spotty dress would suit you better - polka dots are in right now’

When I wore the denim shorts she said ‘you’re brave ! They are very short ! How many years have you had those!’

When I wore the 3/4 leggings she again took me aside and said ‘let’s go shopping one day - you need some wide leg linen trousers not these leggings ! I’m surprised they even still sell them or are they vintage again ‘ and laughed?

I said to her no thanks I’m comfortable and it’s hot so I’m wearing what I like and what keeps me cool. She said ‘come on sometimes we all need a bit of a refresh!’

AIBU to think she was really rude ??? I didn’t feel hurt but just irritated.

I love it because it’s comfortable and cool when the weather is hot so I think I love it for how it fits and feels rather than how it looks if that makes sense. It’s just my go to ‘very hot day dress’ as it’s cotton