Sometimes you just have to let it go. Just let her be wrong. It's ok, you'll be fabulous and she'll be unhappy in her weird thrift combination and she'll take it off. Maybe she can rent a costume, but you shouldn't be fretting so much over her if she's being difficult. It's clothes. Just go have a nice fun day at the faire together.

Edit: I do understand you. My mother insisted on thrifting for her wedding. It was a major pain sewing her dress when I could've probably just made it myself. And she was totally unaware of how much work it was for me.

I love that you and your mom do this and I love that you make your own costumes.

Another commenter suggested letting your mom be wrong. That's great advice. Your mom's bad costume decision is about her and maybe when she sees the difference in the quality of your costumes she will she was wrong. Maybe she won't. Either way, try not to let it bother you.

It sounds to me that she wants to control her own dress, so let her do it. It also sounds like she has zero clue about what sewing entails, that is is actually easier to sew from scratch to the measurements of the person, rather than taking three different things which don't fit neither to the person nor with each other.

Let her be wrong.

I can imagine it's hard to say no to own mother, but you should really not do it for her if it doesn't make you happy and she doesn't even see a problem. You're the seamstress, you know better. If she can't accept it, she can sew it herself.

I ran into something somewhat similar asking Nigel if he wanted me to make him something for the fair. Sure, he says. I show him some patterns and he’s meh. I’m like I’m not making it if you don’t want to wear it, please be honest. So then he’s like don’t make anything then I don’t want to waste anything you make. Like dude I’ll make something but you need to actually like it.

Anyway I’m gonna dress up and he’ll be in street clothes probably. Take comfort in the fact that at least you aren’t going with a man! Your mom at least has a sense of fun and creativity, even if a bit misguided and unrealistic. Maybe try to take pictures (if that’s something you do) beforehand at the house to avoid missing out if she gets hot too soon?

I hate polyester too. I don't like looking at it or touching it. But I find lack of style and/or what I consider bad taste endearing in people. Mostly I just think it's nice that you and your mom do this together every year. I hope you have fun.

Oh dear.

How much spare time do you have?

Can you make her something from scratch that she can put on when she inevitably cannot stand to stay in her polyester outfit?

(I mean, what does she intend? To walk around in her underwear? Or are you actually wearing normal clothes under the costumes?)

She sees the hard work I put in, and with her actions she tells me, "No, I don't want to wear what you made, I prefer mass-produced material that make me sweat!" And can't understand why that upsets me...

I feel you. My mother never listens to me, even about things were I'm the most knowledgeable person in the family. (I'm not as talented as you, but I have theoretic knowledge no one listens to)

My mother is also someone who likes to hunt for cheap second hand clothes, I think it satisfies some evolutionary urge to go hunting.

Perhaps your mother is the same and just kinda wants you to sew something from the "skins" of the ersatz-animals that she is proud of having hunted down. Might not feel as hurtful if you frame it that way.

As for the apron ... I'd sew the embroidered piece onto something else, like patchwork. I have little experience in sewing, so that might be a stupid idea, but that's what I would try.

Can you make her a cotton shift (or buy a thrifted cotton nightie?) as a base, and then have the polyester dress as a sort of over dress, that laces up across the bodice? Then the skirts sort of hang down, open, revealing the shift under neath. Sew the "apron" waist high on the cotton shift so it peeks out.

Cut the sleeves off the sweater, and lace them with a ribbon or string to the armpits of the wrap dress... cut off the embroidery and use it to trip the bodice of the wrap dress.

The cotton will absorb the sweat, and the polyester won't be as bad if it is hanging loose over it and not touching her body.