There are no right or wrong opinions on whether or not you like something and to be fair highschool dxd's story did suck, it sucked bad, the fan service was the only good thing. They had a cool idea but they executed it terribly in the anime. Theres no actual deserved growth in Issei at all, whenever an opponent way stronger than him shows up he is either saved by someone else or just instantly unlocks a new power. Only a tad bit of actual character developement and actual relationship focus. It's filled with power creep but it ramps so fast it's more like a power curve wall with each new enemy being way stronger than the last. Even though it's quality is low due to everything they did poorly you are free to like it, but there are standards to rate shows by an this show scores very poorly for quality.
To be completely honest, the Novels were great and that's basically all story with little to no art. Anime adaptations (specially at that point in time) weren't known for being all that accurate, so I think the show did lack on some aspects, but still second season kinda picked the pace up. A shame it went downwards after that.
My point being: yes, the story is enjoyable and lighthearted, no need to nitpick on the power scaling when it never was the focus of the series. There are far worse contenders even in modern times, but I do agree the anime could be better adapted