Err, all main characters for each game wore different hats except from Hilda and Hilbert which both wear ordinary caps, and Nate and Rosa wearing visors.
Differing hats = customization?
Well, I don't think it could be a sign, but haven't you noticed that, in the preview, the characters were totally different ? Look at all the others, they look like brothers and sisters, with few exceptions.
Red and Leaf > Same colors (red top and skirt, blue pants and top), pale brown hairs, same signs on their hat, same yellowed bag, same eyes.
Gold and Kris > Same red and white tops, same yellow and dark lined shorts, same part-of-yellow hat.
Ethan and Lyra > Same red top, same black lines under and on top of their knees, same shoes's colors, same Pokégear.
(most diffrent to date)
Brendan and May > Same red and black tops, same shoes's colors, same signs on their "hat", same yellow for their bags, they both have gloves, similar hair color.
Brendan and May (Emerald suit) > Exactly same colors everywhere (green, orange and dark), for the tops and the shorts, same signs on their hat, pratically same shoes, same green color for their bags, similar hair color.
(most similar to date)
Lucas and Dawn > Same hair color (dark-blue), same top's color (dark and white), same sign on their hat, same yellow color for their bags, similars eyes, same red scarfs, same Pokétch.
Lucas and Dawn (Platinum suit) > Same winter scarfs, same hair color (dark-blue), same red top, same white color for their bags, same sign on their hat, only characters who are in witner suit (both similar), boy and girl counterpart accessories's colors (blue and pink).
Hilbert and Hilda > Same caps (with the sign), blue top for Hilbert, blue shorts for Hilda, sort of dark clothes for them two, same hair colors, same sort of bag, same C-Gear.
Nate and Rosa > Same sort of eyes, same hair color, same sort of bags, they both have sport pantyhose, same sort of white/grey on shorts and tops, similar wearing visors.
X and Y Boy and Girl > The boy has a cap, the girl has a hat (like Leaf), he's totally in blue, she's in red and in black (that's what we saw), the boy has brown hair, the girl has blond hair. The only thing in common is that they both have glasses on their hat.