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 you didn't want to wear it at all.
Since you're in seventh grade (I'm assuming) you 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 you told her that 7th graders thought that wearing makeup is weird. I'm in 9th grade and the 7th graders at my school wear more makeup than I do. But, that's fine I guess.

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

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

Good luck. :)