Kenja No Mago is for two types of people. Rabid fans of the isekai genre, or people who haven't been introduced to the isekai genre. Why? Because Kenja No Mago is the definition of mediocre. It's both what isekai fans want, and what new readers to isekai should, on average, expect.

The story is utterly average. Self insert protagonist is reborn into another world and ridiculously overpowered. Which isn't a bad thing, it's just unoriginal. The protagonist is liked by everyone and talented since birth, has a lovely love interest, good friends, and evil enemy. There is literally not one ounce of originality in the story. The typical fantasy world, with it's typical magic, and typical evil demons are utterly devoid uniqueness.

The characters are carbon copy cutouts of archetypes of the major character types you'll find in any manga, let alone isekai manga. Again, not a single ounce of originality. The self insert, OP, kind, boring Protagonist. The Powerful, royal best friend. The deredere, cute love interest, who is oh so shy. The evil demon lord archenemy with the tragic past (which is laughable in how poorly written it is). And then the mob characters, who are forgettable at best.

Art is pretty average as well, nothing to write home about or particularly critique. That said, being average in something like entertainment, isn't a good thing.

Enjoyment is subjective, I personally don't have any attachment to isekai or particularly like insert protagonists, so I'll leave this one up to the reader.

Overall, Kenja No Mago is a master of cliche, king of average, and lord of unoriginal. Filled with an OP mc, undetailed world, forgettable characters, and a complete lack of originality. If this is your first time reading isekai, this would be a fine read, and maybe a 7, and if you're a rabid isekai fan, I'm this, like every other isekai is a 9 or 10. But to anyone else, this is utterly mediocre.