If you use that logic, the fact that people mock it, then you can say the Prime Minister is not taken seriously. Nor is the President of the United States. Nor is Al Qaeda, for that matter.
People often mock things that are powerful and take a big role in the general picture of things. This is why such prominent things as major politicians, world organisations etc etc are so mocked. Have I Got News For You feeds off these types of things because people like to laugh at people who they may conceive as superior to them. People like to think that they could do a better job at running the country, neglecting the fact that they themselves have achieved very little in life. This is why you're more likely to hear ''David Cameron is a ****'' than ''Peter Sutcliffe is a ****''. Everyone knows David Cameron is an infinitely nicer person, but a lot of people are jealous of the power he holds and thus enjoy knocking him.
Furthermore, the majority of the people on this site are extremely liberal, unrealistic and naive individuals who see it as fashionable to knock the Daily Mail.
It's just the thing to do. Just like it is fashionable to defend all immigrants. We, as students, just have this annoying trait where we think we're better, and could do a better job, than the people running the country.
Finally, the Daily Mail is not the only paper with a disparaging nickname. How about ''The Grauniad'' and ''The 'Torygraph''???
No doubt you're also one of those people who will use every opportunity possible to scream ''THAT'S RACIST!''