As usual, I had further thoughts about something after I posted, in this case the notion that live-action movies based on comics tend to come off looking more cartoonish that the cartoon. I first said this...
Perhaps the producers equated cartoons with childish silliness, and felt they must incorporate that into the movie, resulting in an over-the-top feel that is an insult to the creativity of the makers of the original material who often tried to avoid doing such things in order to aim for a wide audience.
Later on, a more likely possibility occurred to me: it may be simply a matter of incompatibility. Certain ideas only work in cartoon form, with the attendant natural unreality. Removing that by translating it into live-action form makes the entire plot look silly. The Grinch is a cartoon character living in a cartoon world in which he is just another creature and can go about his business. Putting him into a real-world context turns him into a freak and a distraction from the plot.
The Flintstones is another example. Imagine trying to sell the live-action concept to a studio without the crutch of the successful cartoon - "It's The Honeymooners, but as cavemen, in a Stone Age version of suburban life, with primitive takes on modern conveniences." Most of the response would concern your ability to manage your own affairs. That plot requires the suspension of belief that cames with animation. Without it, you get John Goodman and Rick Moranis running around in fake fur.
Of course some cartoons can translate fairly well, such as Superman, Batman and other characters that are human-based. The key is to make sure the entire world in which the character exists is translated properly. Otherwise you end up with a movie-length equivalent of the brief scene from a past Simpsons Halloween special in which a 3-D Homer is walking down a city street being stared at by everyone.