If you look closely at the logo emblems they actually resemble the shape of their respective mascot legendary: the Moon logo is Lunaala's basic body shape, and the Sun logo is fashioned subtly after Solgaleo's mane. Pretty neat!
I really do love the attention to detail with the fact they made the legendaries look pretty much the shape of their game's logo. It's definitely very cool and a reason why I reacted to them very well.