It's users have a sense of humour, is that why you say the coin is a joke?
That sense of humour and its many uses as something other than a short-term investment are precisely the reasons that people will put money in it; because it doesn't give the impression of being nothing more than a ponzy scheme like most other coins often seem to be. And many people want to invest in something that is used to do good, and Dogecoin has a great track record with the charities.