In a recent interview with Japanese magazine Nintendo Dream, Splatoon director Tsubasa Sakaguchi and producer Hisashi Nogami discussed the limited customization options that are currently available for the Inklings. They stated that they limited the customization options for the Inklings to eye and skin color so that they could establish them as characters; the Inklings are brand new characters, so the developers wanted players to identify them as the same kind of character. They felt that limiting the customizible features was the best way to establish the Inklings.
Here are their full statements:
Nintendo Dream: “‘The more character customization, the better’ is the usual rule in online games. But you can only customize the eye and skin color of your Inkling.”
Sakaguchi: Perhaps everyone has forgotten that Inklings are still unknown characters for people. Inklings are newcomers that just came out.
Nintendo Dream: “Ah, that’s right (laughs).”
Sakaguchi: “I think it’s better to tightly narrow down “it’s this” when presenting a new character to everyone.”
Nogami: “If we suddenly had variation like three hairstyles with braid or bun, people wouldn’t consider them as the same thing [character]. Therefore we limited the customization to the minimum with skin and eye color.”
Source: Nintendo Dream (via Nintendo Everything)