What makes her innate and almost genetically hard-wired feeling of inadequacy worse is that she's lousy at picking up on social cues. Not only did a long-undiagnosed visual impairment mean that she never learned to read facial reactions at all well, Elly's need to sentimentalize a really hoorible time in a human being's life and fear of being bereft of purpose led her to get locked out of pretty much every loop. Add in how Elly isn't really good at giving advice and you can see why Liz really never twigged to what was going on around her. She doesn't know how to read body language and she sucks at looking at the big picture because her mother is letting her own issues get in the way of being a mother. She thus turned to a 'convenient' source of good advice: network television. The problem is that the goggle box is just as bad a parent as Elly and just as likely to fill impressionable minds with mush-headed ideas.
This isn't just why Candice and Dawn had to sit her down and explain that Anthony didn't just happen to show up places where she was because he announced his intention in a way not thought of in the philosophy of Zack Morris. It isn't just why she didn't see that Eric was a player until it was too late. It isn't just also why she didn't admit that the people of Mtigwaki weren't lovers of adultery because they didn't want to get involved in a domestic because they ain't stupid. It's pretty much all the reason why she still sees Thérèse as a soap opera monster who spends her days clubbing seals and plotting the ruin of plain Janes because that's just what people who make Liz feel inadequate DO.
The real reason for this need to see Thérèse as a soap opera monster plotting her ruin is because that made more sense than her rival being....well....her!!! As Anthony's Liography clearly indicates, what Liz will always see as an irrational monster who hates her for no other reason that being a bad person is pretty much a Franco-Ontarian Lizardbreath trying her damnedest to prove herself to her own bloated vermin parents owing to coming a distant second place to a son who was never born at all. Her major malfunction is that she sees a passive little thing like Liz as being a manipulative man-eater who wants HER to die alone and unloved and to laugh at HER misery because she looks like someone who's got everyone fooled into thinking she's harmless because that's how cruel people who want people to die alone operate. Right now, as we speak, the poor woman is telling her boyfriend about the blonde monster with the rich dentist daddy who has a whole town wrapped around her finger just like on TV! Why, she even managed to turn Thérèse's own child against her, she's that devious and selfish!!
Thus does the trick box amplify the horror caused by the narcissism of minor differences. Liz and Thérèse would have found it hard to get along in the first place because they reminded each other of what they most hate about themselves. A medium that proves the worst just made a bad situation far worse.