Well it is the "(Little) Things that annoy you" thread. However there is some merit to your complaint. Having a different menu sprite for each of Arceus forms would help you immediately remember what form you had it in without needing to check it stat screen, especially if you had multiple Arceus in your party for some reason. Also, while Arceus forms are permanent as long as its holding its plate and are simple recolors, for other Pokemon with temporary forms they went out of their way to give all their forms sprites: Castform (who's form is dependent on the weather), Burmy (what form it is is only useful to know when its going to evolve, otherwise its completely cosmetic. Also if Burmy can change what cloak it is before it evolves it makes this further pointless), Cherrim (another weather dependent form), Deerling & Sawsbuck (purely cosmetic and effected by a mechanic that doesn't effect battles), Vivillon (once again cosmetic), Flabebe & Floette & Florges (cosmetic; FUN FACT: AZ's Floette has its own Menu Sprite
), and Furfrou (cosmetic, and it's only that way for 4 days). You could also add Shellos, Gastrodon, Unfezant, Basculin, Darmanitan Zen Mode, Frillish, Jelliscent, Meloetta Piroutte Forme, Keldeo's Resolite Forme, Pyroar, and Aegislash to that list though I didn't since I feel they have a bit of an excuse. And most of these aren't recolors, they involve some work.