That Ditto doesn't retain any shininess when it transforms, despite that being it's whole schitck. It wouldn't be too bad if the older generations had shiny Ditto transform into shiny versions of other Pokemon. You can't excuse "well maybe it's too much effort for the programmers"
But that one makes sense too. According to its Pokédex data, Ditto could only transform into what it sees, so considering that Ditto would also retain the color patterns of what it'll transform into, it would confirm why Game Freak considered not making a shiny Ditto transform into the shiny variant of its target unless the target Pokémon is shiny.