I googled around for the reference art and yeah this is really good stuff. Looks absolutely terrifying and is scaled really well! Probably the best places to improve upon would be either remaking the eyes to a tilted half circle shape or adding angry eyebrows, to make it look more menacing, instead of surprised. Making the mushroom cap in a larger diameter would add a lot to the costume as well.
Probably going above an beyond would be finding a way to give it more of an organic look with sinew and saliva coming off of it. We accomplished this on our coffins by using rolled plaster tape and oil paints.
Again looks great! Glad to see some really out there costumes from lesser known source material!