Nikko is full of steep stairs. This is a picture of Rinnoji Sanbutsudo, which is a temple with Buddhist statues showing the origin of Nikko's religion. Because renovation is going on, we can go up to the 7th floor, overlook the construction and have a view of Nikko city, but there is no elevator for it. You have to climb up and down the stairs.
Taiyuin Mausoleum and Toshogu Shrine also have steep stairs before the gates. Not barrier free at all.
There is a report of a wheelchair person with a help of a volunteer group visiting Nikko four years ago, which shows some more pictures of the stairs.
Even hiring a private car would be no solution. The cars would be parked far away from the Shrines and Temples.
You can use the World Heritage Bus and get off at the first stop, Hotel Seikouen Mae and visit the museum, which shows a short movie of the creation and renovation of the Twilight Gate, but to see the actual one, there is no other way but climbing the stairs.