Is it a good idea to get Jean C1 for future Stygian cheese?

Given your available options, it's certainly not a bad choice. I wouldn't count on Hoyo continuing to allow the cheese to continue forever, though. Eventually, they're gonna get tired of folks making a joke out of Dire clears and start giving these amped-up elite mobs amped up poise resistance, as well.

Are you asking if you should get a constellation just so you can cheese bullshit content in order to get maybe half an artifact reroll every 6 weeks?