Hi there, Futaba here.

There isn't actually a scientific phenomenon where people who grow their hair out to cover their eyes automatically get a harem of chicks. It's just post hoc ergo propter hoc, or "Just because it happened one after another doesn't mean one caused the other". It's a complete coincidence that most MCs in hentai have that haircut. Perhaps covering the eyes makes them more faceless and thus easier to self-insert.

Anyways, the joke is that Joseph Joestar cut his hair like a Hentai MC (even though he has a grey beard when he has Grey hair. Should the OP have perhaps used Paint Bucket to shade the beard in?) thinking that he could get a harem of chicks with that haircut, but it didn't work, so he kicked some random guy in the knee and shouted at the sky "I'LL NEVER FORGIVE THE JAPANESE", causing everyone around him to look at him wondering what sort of life decisions led him to get such a haircut.

Anyways, Futaba out.