Well, although higher IVs are better, it doesn't have to be an absolute show-stopping factor if you plan to use it in competitive play, you just have to invest your EVs more mindfully. When I say the nine shiny spreads, I mean the nine shiny-possible natures and the set IVs they're programmed to have. You can trade for the other natures but the GTS blocks all shiny Jirachis, however there is a way to upload ribboned/shiny Jirachi to PokeCheck but the way to do it requires the use of a program that is forbidden to speak of here and how they get past the GTS that with the forbidden program is unknown to me.
I don't know the exact spread for each shiny-compatable nature but they're all fixed to low IVs, I think the naughty-natured spread has the highest IVs, I faintly recall remembering that it had two stats that were fairly strong.
Shiny Jirachi cannot be uploaded to Pokecheck because the GTS treats them as hacks even though nine spreads can be legally shiny (Gamefreak didn't intend for those nine spreads to exist nor did they expect anyone to get them).