If you are applying decals to a surface which is gloss painted, a clear coat before decalling is more or less superfluous.
A (gloss) clear coat is recommended before decalling a flat painted surface, especially if you are using a decal setting solution. If you apply a decal directly to a flat surface, you increase the risk of "silvering" (or trapping air beneath the decal causing it to show through the clear parts of the carrier film) due to the irregular surface underneath.
Further, if you use a decal setting solution, the irregular surface may prevent the decal from laying down completely before it dries, resulting in wrinkled decals.