If you or anyone you know uses iOS devices, they know this, because “tap the top to jump to top” has been a built in function on all iOS devices for quite a few years now.
https://www.macobserver.com/tips/quick-tip/return-to-top-ios-iphone-scroll/
Maybe your users are all Android users, somehow? 
Anyways, yeah, just click the topic title to jump to the top. Easy and works everywhere on every Discourse instance ever created with zero special install or config required… desktop, laptop, tablet, mobile.