I hope it's not its real shiny form because that'll be funny for me. I mean really, a shiny Mimikyu is different than other regular Mimikyu just because it designed a different cloth (if Mimikyu knows that being a shiny makes people love you even more, I bet all Mimikyu will design themselves a shiny cloth)
I assume its shiny form is its hidden self under the cloth is white instead of black (much like Mega Gengar), or any other colour. That seems the reasonable path to go for GF