Given the ecology written for them, I can see how Sporeon would be Dark/Grass, but wouldn't a mushroom themed Eeveelution be more suited to Poison/Grass? Is that just because there's no canon Poison type Eeveelution yet?
That’s a great question! I have a long answer.
When I conceptualize the dual-types, while I want them to be their own thing, they’re each based on several features of the ‘base’ Eeveelutions, along with their dex entries. For Leafeon, one take away is always plant growth (or in Sporeon’s case, fungal growth). For Umbreon, there are several features that stand out to me - dark coloration, glowing features, a more overall sinister appearance, and its poisonous sweat. These features all combined well into Sporeon, to the point that I don’t think Poison/Grass would fit it better than Dark/Grass.
Pokemon, as a rule, seems to treat mushrooms as grass-types. Most mushroom Pokemon aren’t poison types at all:
So in general, I don’t think mushrooms automatically warrant a poison-type. The dark-type portion ties into the bioluminescent quality of the mushrooms (like Umbreon’s glowing rings), along with Sporeon’s nocturnal nature and general affinity for darkness and shadows, as well as their subjective creepiness.
As for their being no canon poison-type Eeveelution yet, that wouldn’t change Sporeon’s typing in my eyes, but I think it’s still relevant.
I am waiting until we get an official poison-type (though if we don’t get any new Eeveelutions in Gen 8, that might change) before designing any part poison-types, although I would naturally go a different route for a Poison/Grass Eeveelution. There’s plenty of poisonous plants that don’t have such a strong connotation with darkness and eeriness, and I think I could find a better fit.