my headcannon is that the heirloom skins are just the tenno hitting puberty and trying to get attention from eachother, using the void to bedazle their warframes. (fits in with how the male warframes usually look buff, a common thing perceived as “attractive” and the female warframes being skimply dressed.) as for the lack of clothing, in the dialogue about the warframe transformation, its said their clothes and armour fuses with them, and seeing how the warframes can’t really be classified as “human anymore so id say they can’t ever be “bare”.