Hello,
As with any other country, the further you get from the big urban areas, the harder accessibility becomes.
I’ve been to Aomori (Hirosaki to see the castle), Hakodate, and Sapporo. Sapporo should be fine in the main city area as even when I was there 10+ years ago the trains and subway were accessible.
In Hirosaki, there were accessible buses, but not all buses were accessible – for example every third bus on the route etc. That two was about 5 years ago, so there have likely been improvements as new city buses are required to be accessible*, but just be warned that you may need to wait a bus or two.
In Hakodate I used the tram, but not all stops are accessible due to the tram stop configuration and width, and I think some older trams themselves were not accessible but the newer 8100 and 9600 types are. I went from downtown to the fort. It’s in Japanese, but here are the low-floor tram schedules: https://www.city.hakodate.hokkaido.jp/docs/2014012100939/#teisyou – there is a phone number there, perhaps when you get there you can ask your hotel to call to confirm. I stayed here: https://danshaku-club.com/eng/eng-rooms/
Anyway, it certainly is possible – just don’t expect it to go as smoothly as Tokyo!
(*Note: highway/tour buses are still almost never accessible. I’m just referring to the local buses.)