TLDR; she became prime minister (PM) because some powerful forces in the Conservative Party thought she would be easy to control (they were right), she instigated a borderline libertarian, trickle-down tax reform, the international money markets went insane (because it was borderline insane, unfunded and left no doubt as to her ineptitude), the U.K. economy took a massive hit - and all of this within a week or two - and it became crystal clear that she actually was indeed exactly as inept and dumb as she looked, so she was replaced because while everyone in her batshit crazy party was like “tax cuts for the rich”, enough of them understood that for the money markets to have any confidence in the U.K. economy couldn’t actually be run by someone with the intelligence of a lettuce. Fun fact, a newspaper ran a “who will die first, this lettuce or Liz Truss?” live video stream of a lettuce were proven right; the lettuce outlived her.
I’m missing a lot of detail that can best be summarised as: Liz Truss is a shining example of the Dunning-Kruger effect.
Although in fairness to her, she did open up new POOOOOOORK MARKETS!!!