When is South Park going to do an episode on her?

They're kind of weaklings these days.

The South Park guys support her most likely, why would they make fun of her?

Already did, with the recent Christmas episode. Santa Claus lecturing the residents of South Park and taking away their liquor, marijuana, cocaine, etc..

Years ago they did an episode that kind of applies to her.

It was making fun of how adults who are pushing causes will use kids to try and get their message across. The plot involved trying to save the rainforest and stuff.

It's too easy.

Probably out of compassion or pity. She is just an unwitting pawn being used by powers greater than her that don't care about ruining the life of a young girl. Matt and Trey usually save their ridicule for adults.

She was named "Person of the Year" on the same day that the season finale aired. Prior to that, she was probably too small time to even be on their radar. If she has any relevancy by the time next season rolls around, I wouldn't be surprised if they gave an opinion on the situation.

They should just add a young Greta to class next season. The only phrase she utters is "how dare you!". Butters falls in love.