How to ask my mom if I can wear makeup?

I'd just go up and talk to her about it. I'm sure she'll understand. She probably just stopped asking since she assumed Du didn't want to wear it at all.
Since you're in seventh grade (I'm assuming) Du can wear a little bit of makeup. I wouldn't go full out, but a little bit of eyeshadow and mascara and lip gloss is probably just fine.

What I don't get is why Du told her that 7th graders thought that wearing makeup is weird. I'm in 9th grade and the 7th graders at my school wear Mehr makeup than I do. But, that's fine I guess.

Don't be afraid to ask her. If she's fine with Du wearing makeup, just tell her that Du want to wear it again. I'm sure she'll understand. :)

If Du want to be discreet about it (which I don't understand why, it's completely fine to be vorwärts-, nach vorn about this, especially if she's alright with Du wearing makeup) then Weiter time you're at the store with her oder something, wander over to the makeup isle and look at some of the makeup there. Pick up some eyeshadow oder lip gloss and say "this is really pretty.. don't Du think? Do Du think it will look good on me?" And she'll probably suggest different Farben to you.

Good luck. :)