Context: the princess is dead. The demon cleric tells the demon lord how she died.
chiiin ちーーーん *ring of a bell* (sound effect used when a character dies or is defeated, probably originates from the bell used in a butsudan 仏壇, a "Buddhist altar" for a deceased family member.)
doku-kinoko no boushi wo futon ni shitasou desu... 毒キノコの帽子を布団にしたそうです･･･ It seems that [she] made a poisonous-mushroom's cap into a bed.
In other words, she used the cap (the top part) of the mushroom as if it were a bed.
She treated it as a bed.
futon 布団 Traditional Japanese bedding, typically placed on a tatami floor. The term refers to both the mattress and the duvet, but just "bed" is a good enough translation most of the time..
moshikashite baka na no ka? もしかしてバカなのか？ Could it be that [she] is an idiot?
