Aside from the movie just not looking fun. Why does the effect look so bad?.
Like compare it with 30+ years old Who Framed Roger Rabbit. The actual 2D look, the shadows, the lighting....it gets that "cartoons in the real world" look much better.:
Nothing will ever look as good as Roger Rabbit.
Roger Rabbit looks so good not because of the available technology or tools, those were too limited at the time. It looks so good because of the dedication, talent and effort behind the people who made it. It was literally a movie where everyone involved wanted to flex their abilities and make it as convincing as possible, not because they needed to but because they could. They wanted to prove they can make this cartoon+actors thing look good. Also, Richard Williams was involved and if you know anything about this man you also know the end result could either be a masterpiece or a cancelled product. RIP btw.
This kind of thing doesn't really happen anymore. Not at this level:
There are talented people in today's industry, yes, but now they rely much more on technology, ease of use and available tools than making something the hard way and prove they can do it. This is why traditional handmade animation is dying and CGI is taking it's place. Because it's harder and less convenient to make it by hand. CGI can look great, no doubt, but my point is that convenience trumps everything now. There's also no way anyone today would allow someone like Richard Williams decimate the budget the way Richard Williams did. But back then they let him do it because the producers/directors wanted to prove something.
There's nothing to prove anymore, they don't have to prove they can make dinosaurs look like real, they already did that with the original Jurassic Park, now all they need to do is make stories about dinosaurs doing cool and funny things. Same principle applies here. They don't need to prove they can make Tom and Jerry look
really good among the actors. They just need to make a Tom and Jerry movie with actors, whatever the hell that is. They only need to make it look serviceable enough so they can tell the story. Here, take this computer, and make this scene.
And let's not forget how Roger Rabbit is mainly an adult's movie. It's not even a "whole family" movie. It was made for adults in mind. The fact that kids could watch and enjoy it was a side effect of the usage of cartoons. That's why 100% of people who watched it as kids appreciate it way more as adults. Could you say the same for movies like Space Jam and those later Looney Toon movies? Does this movie look like it will be appreciated by future adults who watched it in the same way?
Roger Rabbit is the best example of "lighting in the bottle" i can think of in the movie industry. The right people met at the right time, with the right budget to make the right product. The chances of that happening again for a similar movie in our lifetime are very slim.