I know I'm old and times have changed...

[Athleticpotential]

but is it OK to go to an interview in leggings and flip flops?

It isn't a senior role and there's no particularly formal dress code, although actually leggings are not allowed and flip flops are banned for safety reasons, candidate would not necessarily have known that.

I know what she looks like shouldn't make a difference and maybe it's all she had etc, but surely it's still normal to make some sort of effort to be smart for an interview?

I would say that attire is not suitable.
I understand that a lot of people struggle with money I find it difficult to believe they don't have a proper pair of shoes, trousers, dark ( none holey) jeans and a plain t-shirt / blouse that would have been more suitable.

If I was going for a job interview for kitchen work I would certainly make sure something suitable for in the kitchen just in case they plan to show me around it.

I have been doing on line interviews recently.
When the invitation is sent it clearly says we want your camera turned on.
The amount of people that are wearing clothes that I think are not suitable surprised me. I have had
2 guys in vest
3 people in onesie's , one of with kept the hood ( compleat with dog ears ) up
One with food on his very baggy old looking t-shirt
One with BITCH on her jumper.
Several women that looked like they were ready to go clubbing, sequin tops / crop tops / very thin straps & low cut enough do I could see bras straps / top of cups.

Cartoon pics of dogs humping .

All could be fixed easily with a cheap plain white T-shirt , blouse or shirt only needed to keep them for half an hour.
First impression count and unfortunately all of my first impression with them were not good

I have also had people think it's OK to be out shopping , walking dog , picking DCs up ( everyone is given at least 3 choices of time slots with 3 days notice ) but that's a whole other thread !