Slapstick comedy with a serious plotline just doesn't work.
Let me clarify: Slapstick is pure comedy. Everything is about the jokes. The characters can be as goofy as needed and the plot is lighthearted and can be as hollow and faulty as needed. No one cares about plot holes in pure comedy, since the plot is there only to carry the jokes. The best example is Konosuba: every plot device in the story is meant to carry you to new jokes.
Now the second form is the most common. Jokes in a serious plot. It's what has been done in every Marvel and Romance Comedy movies in decades, but it's also hard to do.
The joke can't make holes in the plot, nor can it break immersion, mainly because the story is a serious one. The joke is used to diffuse ******* and add variety.
In this novel though, we have a serious story filled with jokes that will totally break the immersion and fill it with plot holes and dumb reasonings. That's why it doesn't work and the jokes will come as forced while you lose interest and stop being invested in the story.
Example: The goddess chooses a pervert dumbass for her hero by mistake (the MC). He falls down in the world and more than 2k people immediately accept him as their new king, including an old general. He also uses a donkey as a mount because the other people are using the horses, even in battle and diplomatic missions. The goddess gives the humans an incredible stupid and useless magic that serves only as a joke. Does this seem like serious plot devices to you?
Yet the story deeply delves in wars against races, the human's plight, resource management, tactics to defeat enemies, all in a serious tone. It just doesn't match.
My second complain is the excess of mundane. When I'm reading fantasy I expect fantastic unnatural things(duh), but in this world everything is magical, except for humans. They still walk around just poking things with pointy sticks. They have no magic nor use any magical things. It's boring middle ages settings.
The last thing was pacing. 20 chapters for the MC to get used to the camp and intercept a patrol of their giant-bull-like enemies. Quite alot of chaps wasted on prep time and things that didn't set the plot forward.
I don't know if it's possible to integrate slapstick comedy and a serious plotline, but this is definitely not it.