1. Inept wordplay as evidenced by Liz's thought bubble.
2. Behavior contrary to normal human motivations. Not only did Thérèse apologize to Liz for intruding, Françoise ran to the woman who replaced the mother she was pining for. That never happens in real life.
3. The exposition of a bizarre philosophy by an idiot. Not only is Liz's notion that she's the better mother merely because she's physically present absurd, she's setting herself up for a lifetime of disappointment because this child will always resent her for destroying her real parents' marriage.
There's a fourth, more fundamental, thing that sullies the happy ending: even after all this, we still know next to nothing about Thérèse. That's because we only have the word of people who regard her as an enemy because they fell threatened by her. Her sin, as far as I can see it, is destroying some inane fantasy that two high-school 'sweethearts', who barely had a romance to start with, would settle down and have a happy life together. If it wasn't bad enough that she got in the way of a nothing that that Elly Patterson convinced herself and everyone else was a something, she actively expressed doubts about Elizabeth's intentions, not to mention about the suburban lifestyle. That was enough to turn Gord and Tracey Mayes against her. They have no interest in being fair-minded to her and they are not. Everything they are makes them rally around a man they see as a victim of circumstance: Anthony. The problem with that, of course, is that they're willfully embracing a viper to their bosom. Awfulny, as we all know, is a manipulative little petty tyrant who, by the use of whining and playing the victim card, bullied his way from victory to victory, all the while maligning the hapless recipient of his hatefulness. We cannot, should not, trust anything that comes out of his mouth. Was the divorce civilized? We don't know? Did he deny her visitation rights? Who's to say? What really went on behind closed doors? He'll never tell us the truth and she's too ashamed to come forward because she knows she'll never be believed.