I’ve read that book and given it as gifts. Not only is it an excellent read, the cover is actually quite accurate to the story. A normal dude, a punk rocker, a prom queen, an alien and a bowling ball go on a space adventure and meet aliens. How else would you illustrate that?
As several people have already mentioned, the cover is accurate as to the contents. However, I would say it was also quite accurate as to the whole tone of the story (silly and OTT(but still thoroughly enjoyable)).
In fact, it was the whole silliness of the cover that made me want to read it in the first place. Odd that similar covers for the Discworld books done by Josh Kirby had the opposite effect….