It was a joke then and it's a joke now (sort of).
We say 'sort of' because Trump is currently leading the polls to be the Republican Presidential candidate for the 2016 US election, but El Donaldo has been around for a long time as the butt of jokes and The Simpsons are frequent makers of said jokes.
But it turns out that The Simpsons actually called out the ridiculous idea of him as President way back in 2000 in the Season 11 episode "Bart to the Future", in which a future President Lisa Simpson and Secretary Milhouse have to deal with a debt crisis left by former President Trump.
Let's just hope, for the sake of humanity, that this doesn't come true.