Skincraft characters work in "Minecraft" multiplayer if either all members of the multiplayer game have the mod individually installed or if the host has enabled the mod in the server settings. Under either condition, Skincraft characters function the same way on an online server as they do in single-player mode.
"Minecraft" mods can be either server mods or client mods. If Skincraft is installed as a server mod by the server host, all players have access to the Skincraft mod. However, client mods are dealt with on a case-by-case basis. This means that while Skincraft may appear to be working perfectly, other players won't be able to see the changes unless they too install a copy of mod.