As the article mentions, the North Sea oil boom happened right as she came to power. That meant that a lot of people saw the UK as suddenly having a lot more money right after she came to power. A lot of other people became homeowners through Right to Buy, and became financially comfortable because of it. If no one's told someone the money was going to come anyway and she just pissed it into the wind, and they don't realise that their own home ownership success was at the expense of the next generation of renters, it's understandable that they'd think her premiership went really well.
Like with many things in this country, the cause is the right-wing press selectively reporting on things, so half the country can rattle off loads of things incompetent politicians did well (even if they were by accident or so easy that they deserve no credit) but barely know any of the cock-ups or intentional actions against their constituents' interests. Thatcher in particular is elevated because she acted especially in the interests of people like Rupert Murdoch, and some people experienced things going well while she was in charge, but no Tory leader since has come to power right as something unrelated was about to give the country loads of money, so good things didn't happen to enough people for there to be anything to turn into a hero myth.