Mermaids with hair over their eyes are usually from deeper waters. The sea floor has almost no light, so deep sea mermaids (few of which have functional eyes) visiting the surface would be blinded by the sun.
To mermaids, covering their eyes with hair is similar to sunglasses.
Deep sea mermaids are also shy, accustomed to dark waters for hiding their monstrous appearances.
Freshwater Mermaid: Howdy y'all, I’d like you to meet my cousin. She’s a little shy :) 30ft Tall Deep Sea Mermaid with hair so long it turns the water black: ᴴᵉʷʷᵒ
[ context : the players are in a dungeon with a sentient stone lion ‘Leo’ blocking their only exit. they have to make him say his name without realising it. ]
Gnome Cleric: What’s your name?
Leo (DM): um– well, I can’t exactly… you see–
Half-Elf Sorcerer, *really* loudly: wHAT DO GYMNASTS WEAR????!!!
Leo (DM, completely baffled): uh, Leotards…?– oH FUCK!
*followed by laughter so intense, our half-orc warrior falls to the floor, tears streaming down their face*
