It depends on the size of the girl's breast, but I think girls with smaller to medium-sized chests are more likely to have torpedo-shaped breasts, because they sit higher on the chest and are usually not as rounded-out as large breasts. I think they're fairly common, and are probably more natural than the ideal "bubble breasts".
My boobs are like that. Didn't know they had a cool name like torpedo breasts lol. I'm kind of unhappy with them because I feel like they sag. They're big but they don't look like the perfect sit up perky boobs that I wish I had
Mine are more round than anything, maybe a little shallower on top than below. A lot of women with fake breasts, especially badly-done ones, end up with this shape. But if it's what you prefer, I guess there's something for everyone.