Unlike a certain robot, Mimigas seem to be quite susceptible to injury. And while mushrooms are... relatively harmless, they still pose a threat. Imagine just sitting there paying your respects when mushrooms start popping up around you. "Ha ha... Mushrooms." *SQUISH*! *powdery poof* -1 *You've been poisoned!* "IT stings...!" And that's when you trip into a giant mushroom and fall over... and the "gravekeeper" comes for you =(
*STAB STAB STAB
(Really what was that thing?! The... gravekeeper. His CS+ text in the credits say "Slices you up". ............ maybe he's a ghost/reaper. I feel like he's going to haunt Quote. Like that Spelunky ghost did.)
Anyway, the mushrooms were probably a sign of unrest. If the Mimigas were superstitious, mushrooms are probably a bad sign, especially if the graveyard normally looks like it did in the credits. Besides if they are being kidnapped; nobody is going to have time to deal with it. And who would want to go into a graveyard when you're in that situation...
Perhaps another theory is that a certain mushroom is the root of all these problems. And that one *Is* tough. (And unreachable?) Or the Demon Crown simply made the entire island's fauna and flora extra deadly.