Skill Decals are decals you can attach to a Fighter's body and it will give a vast array of buffed effects. They can be placed on your Fighter's body by visiting the Mushroom Club in the waiting room and selecting "Change Decal".
Whenever a Fighter dies however, unless you pay to salvage your Fighter, the Skill Decals on their bodies will disappear. You will also lose them if you peel it off your character for more Skill Decals when changing them.
To obtain Skill Decal, you can either get them from login bonuses, from Uncle Prime, or buy them from the Mushroom Club.