One popular 'isekai ojisan light novel' could be 'Isekai Ojisan: Isekai ni Tensei Shita Kedo Cheat Nanka Shinai yo'. It tells an interesting story about an uncle who gets isekai -ed but doesn't have the typical cheat skills.
I'm not entirely sure as I haven't read it in detail. But generally, it might be about some sort of fantasy story involving a bishoujo (a cute girl character in Japanese media) and an ojisan (uncle or older man) in a novel - like setting.
I'm not sure specifically what 'Cap deage' means. It could be a very specific term, a misspelling, or something relatively unknown. It might be a proper name, a local term in a certain area or a very niche concept.
I'm not sure who the main characters are exactly as I don't know this work well. But from the title, it seems that a bishoujo and an ojisan would be important characters.
I don't have enough information to say exactly who Cap'n Kathy is. She could be an original character created by the fan fiction writer. Maybe she's a pirate - the 'cap'n' part could suggest that - who has some connection to the wizarding world of Harry Potter.
The cap man and monkey story is an interesting one. There was this cap man who was traveling around selling his caps. He reached a place near a forest and decided to take a rest under a tree. While he was sleeping, the monkeys in the tree noticed the caps. They climbed down, grabbed the caps, and went back up the tree. When the man woke up, he was confused at first, then saw the monkeys with his caps. He was worried as those caps were his livelihood. But then he remembered that monkeys often mimic human actions. So he took off his own cap and tossed it. The monkeys, being the imitators they are, did the same with the caps they had taken, and the cap man got his caps back.