Do you feel like cosplaying your favorite Disney character? Is this character dark-skinned or differ from your skin color?
Go for it! Your character will be awesome regardless!
Wait, do you feel like tanning or “coloring” your skin to be more “realistic” to the character?
JUST STOP. Nope. Nope. Nope.
You are doing it wrong.
This is called blackface/brownface/yellowface. It is disrespectful and has been used to marginalize and degrade many cultures throughout history. There is never a correct time to do it, there are no free passes and you can’t say you “didn’t mean it that way”. Just don’t do it.
A good cosplay is not a matter of skin color, it should never be.