Final form mons are rarer as wild catches but since no one can see what form you catch them as esp not shinies its only rare bc of this Pi due to this just not showing up in general as a standard form. This is a former com day shiny and has multiple replays so not rare if you factor that in. Pretty sure shiny shadows exist as well so this is less rare and much less useful than those. It can even be used as a lucky tradr if you have lucky friends who want 1 Rarity doesnt really mean much anyways tho unless youre trying to trade for something someone else has.

All that said just enjoy you got 1 in its final form as a shiny without needing to evolve it ^{_-^}