Does Whitebeard's mustache, which looks like the moon, have any meaning?
According to the wiki,
Later in his life, he grew the prominent crescent-shaped mustache, which would spawn his epithet, "Whitebeard".
Well, there's no canonical reasoning to it as it seems. But in mangas and animes the animators and/or mangakas usually make certain characters (especially the important ones) in such a way that they stand out from the rest, to make an impact of that character on the fans/viewers/readers minds.
One more reason I thought of could be that the mangas are made in black and white. So there are no colours to support the uniqueness of the characters. So the mangakas rely on the visually striking exotic characterisation of the characters and that has actually worked! It's a psychological successful experiment to make the colourless pictures worth remembering. The mangakas try making most unique characters without violating the copyrights of other characters' appearance from other animes and without making it look like a cliché! (He is playing with your mind and you don't even realize it!)
It's one of the main reasons that animé characters and animes appear weird to the people who don't watch them because the characters are so unusual and unique!
You can figure it out yourself: think of a character and what comes to your mind first (let's say from One Piece and ignore the coloring of the characters):
- Luffy: Straw-Hat, scar below the left eye, round-eyes
- Zoro: Bad-assness-overflowing, muscles, three piercings, three swords, bandana wrapped around arm
- Nami: Tattoo on left arm, perfect body (well... after timeskip)
- Sanji: One eye covered with hair, chivalrous getup, spiral eyebrows
- Ussopp: Long-nose, crooked body (before the timeskip)
- Shanks: No Left Arm, three scars across left-eye
- WhiteBeard: Huge, well-built body, black bandana, his crescent-shaped beard (don't use "moon-shaped" please)
I can go on and on for all characters across all animes but this much might suffice...
Hope this helps and proves reasonable.