I've had horses rub the mane off like that. I don't have clippers but have lots of straight edge razors. I cut it all one length with the razor. I was a hair stylist in my youth so I'm pretty good with a straight edge. If your not don't try it.
Here's my Appy. His mane stands straight up, and never gets longer 5 inches, but flops back and forth when natural, so I roach it. It's much tidier this way!In this pic, it's actually a little grown out, but still looks okay.
I would either pull it or cut it with thinning sheers if you want a neat look. Don't use regular scissors. They give a perfect cut and it looks bad. I personally think it's really cute the way it is and you arent going to be showing anytime soon... Why don't you just put MTG in it and let it grow out, once it gets to a decent length you can pull it to all be the "same" lenght.... course... I like it, gives her character. If she were mine... I'd probably just leave it... unless I was showing... which you arent this year.....
I was going to say roach it until I read all of the replies. I actually like the idea of pulling all of the longer hair to get closer to the shorter length, and then repeating when everything grows out a bit. I would then use MTG on a regular basis, and keep it banded to train it. It is amazing, even if you don't even the hair out completely, what a difference it will make to get rid of the long, scraggly hairs.