Mature is usually used for art that is adult, but not graphic, like nude paintings, tasteful nudity,  pretty much. :) For example, the portraits in "Far From Perfect" would be considered mature, as it's a woman in a shower, wet clothes, but it's done for artistic reasons, as you state in the description that you were trying to evoke a 1990s vibe. Only reason I used it is I see it in some art communities I follow, where they delineate between general, mature, and adult/NSFW regarding subject matter. 
Hope that clears it up. :)