Interesting.
I think the reason for the "joke", and I use the word advisedly, is because Thatcher became entirely emblematic of a certain approach to life, i.e. free market capitalism/ classic liberalism on the economic side (e.g. reduced trade union power, privatisation of state utilities, sale of public housing) and what would be widely considered a neo-conservative approach to foreign policy (e.g. closer alliance with the US, Euro-sceptiscism, increased nuclear spending).
Some people agreed with this approach, some people hated this approach. Their are, however, very few other politicians who have become so individually identified with a certain policy approach as Thatcher did - we still refer to "Thatcherism" today. We certainly don't refer to "Footism" or "Heathism" or "Wilsonism".
For me, "Thatcher" is increasingly less of a person and more of a "Thatcherism" movement/ sentiment/ policy approach, particularly as time passes. And when people speak strongly against or for her, they're speaking for or against the policies of which she was emblematic.
Not saying that this justifies what is obviously a very low quality joke but maybe explains why a comment like that would draw less flak than it would if levelled against other individuals.