The improvement only happened for some people. Thatcher made it better for a group of Tory voters at the expense of Wales, Northern England, Scotland and quite a fair chunk of the Midlands to boot.
She is adored in some of London and all the Home Counties where her polices led to increased wealth and life outcomes.
In the rest of the country, she is the person who destroyed communities and the fabric of what it was to be British for many.
This is not cognitive dissonance. It's different experiences by different people.
Given this is a thread about HRH Queen Elizabeth II, it's worth noting that she herself and her family were no real fans of how Thatcher conducted herself in relation to some of her policies that were _actively hostile_ to many working class communities.
When the Royal family quietly whisper that they think someone is a snob, well... that's saying something, eh?
The 80's weren't crap. The 80's saw a massive increase in wealth for the working class. There was even a whole comedy character about it - https://knowyourmeme.com/memes/loadsamoney
Yes. The Thatcher era marked the end of a lot of stagnation, and while at the time people might have linked her and the era, plenty of subsequent people don't like Thatcher despite the marked increase in quality of life post her era.
Yet Thatcher is basically remembered as the devil.
I can't fathom the complete cognitive dissonance of people who believe these two things at once.