Thanks for the thoughts. (Hope it's clear I have no connection to SciRate.) The link to comments is directly under the Reddit-style rating, although it only appears if there actually are comments. Most papers don't have comments and you have to be logged into comment.

Personally, I don't expect it to be nice on mobile and I don't want it to be effortless to comment. The amount of work it takes to make a useful academic comment dwarfs those tiny frictions.