Yes, there may be a large amount of undescribed fungal species diversity, but most of it doesn't take the form of distinctive, visibly novel macroscopic structures. A lot of new species descriptions are of cryptic species, species only distinguishable to an expert eye, entirely microbial species, unremarkable little brown things, species only known from sequences in enviromental samples, and so on. I have spent years rambling around tropical forests and have only had a handful of "what the hell is that?" moments that turned out to be truly unusual.
The first time I found a bird's nest fungus in my yard I felt like an explorer. Same for watching the various slime molds that appear and disappear at various points of the year.
I caught a ladybug in the house and while taking it outside I noticed it had something on it, best I can tell it was some type of Laboulbeniales, a parasitic fungus. https://en.wikipedia.org/wiki/Laboulbeniales
If they were obvious they would already be known would they not?
The world 'remarkable' implies that people will be hard pressed not to communicate about it. You don't see an aminita muscaria and not tell someone about it. Clown red mushroom with white spots? Of course you're gonna tell someone, and of course it will have a name if it didn't already.
There are "obviously" new species that do still crop up because undescribed biodiversity is concentrated in little-surveyed tracts of remote tropical rainforest, and the visible spore-producing structures of those fungi might only appear rarely under poorly-understood environmental triggers. So no, they are not all already known about. "Obvious" species new to science are still being described from such locations. Two examples from recent decades I have seen myself, Chlorogaster dipterocarpi, Spongiforma squarepantsii... I know of many more from taxonomic journals. Also I have found some spectacuar mycoheterotrophic plants in the genus Thismia that were only very recently described. Also, there is a high diversity of secotioid hypogeous fungi many of which are "obvious", but you have to be digging around in the soil looking for them to find them, so they are still poorly known.
