I get what you're saying OP. It's not about the clothes per se, but the meta-message the gift is sending.
Yes, babies grow out of things quickly. No, from a purely practical point of view there's no point in spending money on baby clothes as babies don't care what they wear. Yes, it was kind of her to think of buying baby clothes at all. Yes, secondhand is the more environmentally friendly option.
But, BUT, it's not just about all those factors, is it?
This is the first and might be the only daughter OP ever has. She (like all babies) is precious and perfect and brand new, and there is something intangibly lovely and appropriate in honouring all that newness with something new to put her in.
Or something previously worn but special in its own way, because of its provenance (eg christening dress of family member).
She's got the whole of her life to wear secondhand clothes, I do understand the weirdly primal need the OP has to put her new baby in new things.
(That said, my poor little short-changed 4 month old DTSs are dressed exclusively in the sleepsuits I lovingly washed, ironed and put away when DS1 grew out of them.
)