Not all, but a large proportion (13 out of 29 mythics are divine). By comparison, the next most common is Daemons with 8, then Knights with 3. Many troop-types have only one, but even Dragons have only 2.
I get why it makes sense that a large proportion of Mythic troops would fall into these two categories, but this comment is on point:
Like you, I don’t really feel like it is ruining the game or anything. I’m not afraid to face these divine teams in PVP, but I know that if they have Mercy, there is a small chance that I might get a board that I can’t manage, leading to a certain loss. Many other teams can do the same thing to me from a bad starting board (Psion/Famine, ForestTroll/Nyx/Mab, Goblins, etc.), so I’m not on the nerf train, but I do think there is a balance issue here.
Edit: I see that I’m on the verge of derailing this thread. Sorry about that.