”Fussiness” over clothes can be indicative of sensory issues - my DD appeared to be “fussy” over clothes when she was younger and would literally only wear leggings and soft t shirts outside school (and often the same ones over and over).
We now know that she is autistic. No amount of “telling her that this is the uniform and she has to wear it” would have ‘cured’ her of these issues and clothing is a daily cause of upset. None of this was apparent before she started school because she could obviously choose her own clothes at home.
If your DD might be experiencing sensory issues (rather than this being just a preference, for example), I would firstly look at what the uniform requirements are and how they might be adapted and then discuss this with the school.
Our school is thankfully quite relaxed about uniform anyway, but my DD wears a stretchy plain white t shirt and cycling shorts, with a soft grey jersey skirt on top for non-PE days (all of these were from ASDA). She would not be able to wear gingham or the hard teflon-coated dresses etc.
You can also get jersey school t-shirts (rather than the hard polo material) in all of the supermarkets which are softer and more like a t-shirt.
If there are sensory issues at play, schools are required to make reasonable adjustments even if you don’t have a diagnosis, so you can discuss this with them.
Otherwise, it it’s more of a case of her just not being used to uniform, you could take her shopping to pick some items herself and try them on in advance of school starting so she has time to get used to them?