I took it another way, but it may be based in having the context of the previous season.
Last season, Heidi and Cartman both felt ostracized. Heidi legitimately, since she "quit twitter" and was basically treated as if she killed herself and was now a ghost. Cartman, because he had all his electronics smashed and so he started acting as if he was was killed and was a ghost. They ran into each other and, since Cartman was acting very progressive and good all season long, got along and started dating. So the relationship was born out of this issue of Heidi being lost and alone, but finally finding someone who seemingly understood her.
Fast forward to this year and Cartman has been very different. Constantly passive aggressive and abusive to her, while acting like he was the one being abused mentally by her. It's been a bit of a background storyline throughout used to make a few jokes on different topics throughout the season. So how does this pertain here?
I think there's two messages. First, comparing Trump supporters to women in an abusive relationship. Those who fall into them usually have something missing in their lives that the abuser seemingly fulfilled early on. Due to this fulfillment, the woman will also overlook the glaring problems that come up later by trying to focus on those good qualities that filled an empty space, no matter how rare they are now in the relationship.
The second kind of speaks to both as well. I don't think they were trying to suggest that criticizing them just entrenches them. RATHER, I think it was poking at the notion that mocking, insulting, and attacking someone for their past choices in an abusive relationship, especially as they're just beginning to come to the realization of how unhealthy it is, will likely re-entrench them. The same goes for similar over the top statements about the other person in the relationship. Why? Because people don't want to feel like an idiot, they don't want to feel dumb, and people tend to be naturally defensive. Even if you're attacking the other person in the relationship, the abused individual still feels like you're calling THEM an idiot because if the other person is so bad what does it say about themselves if they were with them. This leads to naturally defensive responses, which causes the person to cling back to the "good" qualities of the abuser in order to justify and defend themselves. Once they start going back down that road though, it returns them to the cycle of excusing the abuser and returning to that broken abusive relationship.
The girls at the table, for instance, weren't just criticizing Heidi. They were making fun and belittling her for dating Cartman, who they were also making fun of and belitting.
TL;DR - I don't think it was so much "don't criticize". Rather, I think it was suggesting that Trump and his supporters are similar to being in an Abusive Relationship, and that with abusive relationships, mocking and belittling people as they begin to finally realize the relationship isn't healthy is typically going to simply drive them into a naturally defensive position that causes them to stubbornly fall back into that abusive relationship.
That's the thing with Parker and Stone, when they make a message it's rarely just aimed squarely at one side of the equation.
