It honestly wasn't a bad film, but it wasn't a good one. The ending could have been MUCH better, the beginning was eh, and the middle . . . it was eh as well. SPOILERS AHEAD (I mean, obviously)-
Scooby and Shaggy's relationship was portrayed very well, other than the fact that they argue. Fred, Velma and Daphne I feel were BARELY in the movie. They honestly felt like an afterthought. The 'Blue Falcon' aka Brian was honestly a terrible character. I hated him, but not as if the movie wanted me to hate him, I just downright hated him.
And as for the ending . . . oh god it was just bad.
It was too easy to bring Shaggy back from the underworld. I honestly loved the idea of Shaggy and Scooby having to choose who would stay in the real world and who would go to the underworld. It was great! But that was all thrown out the window when literally two seconds later Shaggy comes right back!!
The movie needed a bit more time to be thought through. I get it was a kids movie and honestly, my younger brother loves this movie, but for people my age it just doesn't sit right. Maybe just a month more of thought and it would've been much better.