I guess it would look a little 'odd' assuming it was properly hot, and not just (as sadly often here in England!) that it wasn't really warm enough for usual beachwear!
I very much doubt I'd think anything more of it. But I guess I'd think it a shame if the cover-up was through lack of confidence - we're all different, and it would be nice if everyone felt confident and able to 'reveal' as much as they wish without regard for the mean-spirited, narrow-minded and judgmental among us. Bottom line - any girl (or guy) is free to wear (or not wear) whatever they like to the beach in my view. Perhaps with the caveat that everyone should keep their bottoms on. Topless males or females, total non-issue. Bottomless males or females, hmm, no ta. Gosh, I'm one of those narrow-minded judgmental people too it turns out - how depressing! ;)
As a man who walks on the beach wearing steel toe-caps and a trenchcoat I fail to see what the issue is. It's too bloody cold and spikey to go near the water anyway unless you are looking for whitefish or empty crisp packets.
There is nothing wrong with wearing non-revealing clothes but I would wander why she is at the beach if she has no intention of swimming in the water or getting a tan. That's like joining a dating site when you are only looking to make friends or going to church when you don't believe in God - I'd wonder why they are there.