Here are links to the app, but I’ll note the Discourse guys say on their own website:
Mobile and Tablet
Designed for touch devices from day one. Automatic mobile and touch layouts that scale to fit your device.
And Leo’s comment also stated it’s working fine in a mobile browser on his phone. My preference is, if there’s not much use to installing an app when compared to using a browser, why install the app? (Comments on the Google Play Store imply the app is just showing the mobile version of the page, anyway.)
Apple Appstore: https://itunes.apple.com/us/app/discourse-app/id1173672076
Google Play Store: https://play.google.com/store/apps/details?id=com.discourse