This is because she has done certain things that a snippy, defensive, sullen and just plain angry basket-case like Elly would see as war crimes:
- The first big arc with Thelma ended up with her conning Elly into taking in a puppy she didn't want because she knew she'd get stuck with it because she liked children who needed to learn to obey their mother more than Elly.
- Thelma used to waste Elly's valuable time about boring people she never met, boring places she can't visually imagine and boring events that mean nothing to her.
- Since Thelma is childless and thus possessed with the wicked urge to overstep her authority, she used to override Elly's parenting decisions and spoil dinners for children who had to learn to eat only what Elly cooked when Elly cooked it.
- Thelma has annoying interests that repel Elly and the overweening need to force Elly to partake in them.
- When she explains this to people, they all defend Thelma and call Elly a prickly annoyance who's still an irritant and spoiled brat.
This is why the last time we see Thelma, Elly takes Lizzie with her so that at least someone going to the old folk's home will be happy to see the old meat-axe. The interesting thing is that since she isn't actually family, a form of stupid magic takes place when Thelma passes away. Suddenly, a live irritant will magically become a dead mentor Elly always loved.