Funny characters has nothing to do with the quality of story. They're not mutually exclusive. You can very easily do a Golden Child film with serious subject matter and completely played straight save with horrible things happening, but with a wise cracking protagonist.
As for "fantasy fans", they don't really matter. A fantasy film needs to have widespread appeal, because of the high production cost. You can count the number of successful fantasy films on one hand. "Fantasy fans" aren't really a thing as it's not a widespread genre or recognized audience.
The film needs to be fun. It needs to be watchable. It needs to convince people they will gave an enjoyable time watching the movie.
The catch being, making D&D into a great story is tricky. You're making a brand new plot that has to compete with every fantasy film or novel from scratch. D&D has the setting, but few characters and stories, and most if its novels are frankly terrible. Making a film straight is very, very likely it to fail, being cheesy at best and a higher budget version of the 2000 movie at worst.
The setting of D&D is also super generic. It rips off every fantasy trope out there, making stories told there seem innately unoriginal. Unofficial Lord of the Rings sequels or Game of Thrones copycats. The generic world is a plus for the game, allowing people to play characters simmilar to beloved favourites. But it's detrimental to making a movie.
D&D doesn't have the familiar story of an adaptation and doesn't have the advantage of a unique world. So there's no "hook", nothing to advertise the movie and sell it to audiences as unique. It just becomes "that movie based on the game". Like Battleship.
After all, we already have three Conans, Game of Thrones, a metric effort-tonne of Tolkien. We don't need more of the same.