I agree with a pp who said it's a difficult decision we have to make as parents. Do we let our kid risk being judged and/or laughed at in order to make a stand against gender norms? Personally I wouldn't, but I don't think that means people who do are wrong.
To me, letting kids choose their own things is a lovely idea, and I let me son choose his own things. But, as a parent my desire to protect my child from any kind of sadness - including bullying and ridicule - means that I'd sacrifice the idea of smashing gender norms to protect him from any risk of being picked on.
As a kid I was bullied so this is just something so important to me - and I also know that small kids like SO many things! If my son was drawn towards pink sparkly wellies, I'd never tell him he could not have them. I would, however, excitedly find a less obviously girly pair and if I found another nice pair with something else he liked on them, and showed excitement about them, then he would probably also get excited and decide he wanted the new ones! If the pink sparkly wellies were the ONLY thing he liked then it would be different, but most kids like sooo many things. They can easily be distracted, it's not like they'd massively miss out from not getting the first thing they picked out.
I will never teach my children that pink is only for girls and blue only for boys, but I would try to gently guide my son away from an item that was very OTT and which I thought would get him laughed at. But this is just my way of protecting my son. Other parents might just say yes, wear the sparkly pink boots! And that would be their way of doing what's best for their kid, and that's fine too. I don't think there is a right or wrong, there's only different.
But having worked with young children for many years, I don't think it's correct that little children wouldn't notice something like this. They very often do, and if anything, the younger the kid, the funnier it is when a boy wears something traditionally for girls!