Isekai Ojisan is a classic gimmick story where the author thought of a good concept, ran with it and got some positive feedback, and decides to do the exact same thing forever. The main idea is that the MC got Isekaied into a typical harem fantasy, but whenever some girl show him affection, he couldn't catch them and just continued to do his own thing.
This is basically the whole manga. There is no development in character, story, or romance (except for a couple who has nothing to do with the isekai plot). The story is just the same said concept repeating, and the characters are just like your usual trash isekai characters.
The only gimmick story that I've seen work well is One Punch Man. However, that series was successful because it introduced a relatable cast with parallel themes to the protagonist and decreased the frequency of the gimmick. Isekai Ojisan, unfortunately, fails to achieve this, making it difficult to enjoy beyond the first few chapters.
6/10