If they don't take this seriously, then they really are dumb. They're trying to get drafted high, or put another way, to make the most money. Who wouldn't want to help maximize the amount of money they could make? Now, I don't expect them to cram for the weeks leading up to the test, but no one benefits from getting a 4.
That said, I don't know to what extent GMs use Wonderlic scores in their pre-draft evaluation. I assume they do considering the test is still administered.