Biologically speaking, mushrooms are closer to animals than they are actual vegetables. Along with that, mushrooms also contain protein, Vitamin D, and B vitamins which are also commonly found in meats.
From a gameplay stance, protein can already be tedious to get, even with multiple sources. Mushrooms could potentially act like an alternative or secondary source for protein. Obviously it would be inferior to meat, but still providing enough to be somewhat viable.
Another way this could be remedied is to make mushrooms give both protein and vegetable nutrients, which would be consistent in how they are commonly grouped with vegetables for culinary and nutritional purposes while also being closer to animals biologically. Either way, mushrooms do contain protein, so the fact they don't provide it as a nutrition feels odd. It's not a major problem, but it would benefit immersion if this were added.
Edit: After much discussion, a few things have come to view
- MAKE HARVESTING MUSHROOMS FASTER
- The nutrient system works, however it does get murky with things like mushrooms, so possible changes to that
- Add drying foods to the game