It used to be that any part of a bra showing was not acceptable, even the straps. It was important to have all of it covered, and the color should match what covers it so it does not show through. That was the standard when it was considered underwear, and underwear should not be seen.
Things have changed, especially for you younger people. The bra is still underwear, but it is considered by many to be exposed if it meets whatever fashion they are wearing. Straps are almost always showing, even in many offices. The color often complements rather than matches the outer wear so that it is even more obvious. I see many women wearing low back dresses with a normal bra strap across showing high above the lower point of the dress. Often girls wear the equivalent of "wife-beaters" with just a bra underneath.
Also, I often see "whale-tails" (back top of thong) sticking high above the pants/shorts. I think that is not considered as acceptable as the bra showing. And with all the tight clothes, panty lines show through and girls go to thongs to avoid them showing.
A lot of fashion rules are changing, and they shall continue to do so, but I think it has reached the point that at your age the bra showing is acceptable.
for some odd reason i thought i saw the question as what do you think of invisible bras. haha. wouldn't that be cool. to have the comfort of a bra, but have the look of no bra, they really should make that for you girls. anyway. i can't answer the question since im not a girl but as a guy, why wouldn't i want to see her bra. its her BRA!
It depends, most of the time it's fine I think... but if the bra doesn't fit or is dirty/digging into her back or shoulders, then no. Also, visible bras at a park or running errands or whatever is fine, but not at a formal party or in the office.
It really doesn't bother me. I just see it the same as any other piece of clothing. What makes a bra so different than a crop top or a bathing suit top. There are a lot of girls who walk around in the summer with shorts and bikini tops on.