We don't have a "top" button on every reply, but the "location trail" bar is stuck to the top of the screen now at all times on mobile. In that bar, you should be able to see the title of the thread that you're on (if the thread title is off the screen to the right, you can drag the "location trail" with your finger to the left to see it).

Tapping the thread title will jump you right back to the top of the page.

Edited to add -- this also applies to the "location trail" at the bottom of the page too -- clicking the thread title there will take you to the top. Also, for iPhone users, you can simply tap the phone screen just above the address bar and it will jump you to the top of the page.

Didn't know about the iPhone trick, thanks!

Not as quick as a button, but both fine solutions as long as you have a decent connection.