There's no specific percentage. But honestly I'll say due to how its marketed and the less and less people owning PC's compared to users, and my own observations of readers who come onto the discord.
I'll say roughly 80% uses the app and most aren't even aware tapas has a website. Most just think is an app only.
Which is why a lot of people rather not get booted from the app by breaking apple's TOS and lose a huge percentage of readers.
So yeah it is a big deal if readers can't read your work on a smaller screen.
But on its still a good idea to have it readable on both devises since there are PC only users raises hand