Is it weird for a flat-chested person to wear a 32aa padded bra?

It would look really silly. You could probably just wear a plain white training bra, or white pasties, or perhaps a thicker camisole. Also, don't worry about your breasts. They might grow larger or they might stay small. Either way, some man/woman (whichever you prefer, or both) is going to find them attractive, because there is huge diversity in tastes. I know a number of men who like their women small-chested, and TONS of lesbians with the same preference.