Clothing has always been a bit of a touchy subject between us
to be honest, clothing can be a bit of a touchy subject between anyone.
here's what i've learned after watching a couple of good friends often wear things that didn't do them any justice at all. one friend with a rocking body who insisted on dressing primarily in overalls and sweaters that three of her could fit into. another friend with a likewise good figure - though with a blocky waist - who consistently wore things that only accentuated her blocky waist and as a result, made her always look five or six months along.
both of them occasionally struck gold, but for the most part, their clothes did them a disservice. my gentle suggestions of fashions that might flatter them more were met with... not anger or anything, just disinterest.
because you know what? they liked their clothes. they felt comfortable and cute in them. yes, i would've loved to see friend A wear things like figure-flattering dresses and boots (she's built like a brick shithouse, with pretty legs to boot) and friend B wear things with an empire or A-line in order to flatter her lovely cleavage while narrowing her midsection, but in the end, i realized they LIKED what they were wearing, they didn't feel they needed my approval, and so i stopped trying to mentally 'improve' them.
eventually, over the years, both of them figured it out on their own, and each naturally developed a fashion sense that was far more flattering to their figures.
what these two friends have in common is that they both had confidence issues, just like your girlfriend does. as they became more comfortable with themselves, they organically started wanting their outward appearance to reflect it.
what i'm telling you, in summary, is that while i understand where you're coming from and don't think you're an asshole, there's really no delicate way to approach this. your girlfriend likes her outfits. as fashions change, so too likely will her taste, and you may see 'improvement'. for now, she's selecting these dresses, skirts, and jumpers because she likes them and probably feels cute in them, and yes, you suggesting otherwise is only going to rock the confidence that you say is already lacking.
you say you aren't trying to change her, but you kind of are.
i will say that there have been many times in my 18 years with my husband that he's gently said, 'i know you really like that outfit, but honestly, it doesn't do a whole lot for you.' i never really get angry or upset about it, but that's because i don't generally have any confidence issues. and there have been times where i was thankful to him for telling me, because i truly didn't realize; there have also been times where i've responded, 'yeah, i know you're right, but i like this dress; sorry!' i don't think your girlfriend is at that place, yet. the best thing you can do is probably to just keep quiet and love her for everything she has to offer beyond her clothes, and perhaps occasionally BUY for her one of those outfits you think she'd look great it, with the caveat of, "i saw this and that it would look awesome on you, but no worries if you don't like it; i'll just take it back, no problem." that way she has an out, but you've also put the concept in front of her. and who knows, perhaps once she actually gets the outfit ON, with the understanding that you won't be mad if you have to return it because she doesn't like or feel comfortable in it, she might slowly start to share your taste.