I'm 100% sure you can see the different stats on your Mega Evos. However, PS! doesn't show the different stats for a Forme change; when switching to Blade Forme, hovering over Aegislash will still display its Shield Forme stats. While I do believe this is a thing that is on the "fix this" list that Zarel has that's longer than Ron Jeremy, fixing this has a rather low priority, I'd assume, as it's solely a cosmetic repair for a relatively-new function. There are considerably more important things on Zarel's plate.