Based on the hit cartoon series, Scooby Doo puts the Mystery Inc. gang in a theme park popular amongst spring breakers, where the owner believes voodoo is at foul play. The rest of the plot is still kind of a blur and doesn't quite make sense by the end, but hey, I guess don't overthink it - there's still all those simply *hilarious* food jokes in there!
So Scooby Doo's not perfect. The jokes, much like the series, don't particularly work but can amuse kids and fans of the original series. What does work are the performances and the surprisingly good chemistry between the cast, compiled of extremely forgettable late 90s teen icons. The cast holds true to the characters they're playing very well, and, while their actual performances are overdone and outlandish, perhaps that's best noting the cartoon is so crazy and fastpaced. The film does an outstanding job at making the Scooby Doo series into a film that doesn't go overboard that it's a film, it feels like a mixed up episode of the series. Scooby Doo's other big con, though, is that it's aged pretty horribly in every aspect you can think of. The cast, the cameos, even Scooby himself has poorly done CGI, even for 2002 standards. Scooby Doo is undeniably fun, and it pains me a little to say it, but I enjoyed the film. It works, and for that, it's a decent flick.
What I think the problem is, is that people (and again the critics) have a vision of Scooby Doo in their heads. An elevated vision. And thus, when they view this movie, it doesn't match their view and so they rate it as bad or low. Scooby doo was a pretty simple cartoon and very formulaic and the movie follows that nicely.