You will soon have the option to sort the community feed so that recent posts show up first, which makes your community feeds easier to scan. This feature is expected to roll out in late summer of 2021.
When sorting by Recent posts, any new comment on an older post does not bump a post to the top of the feed. Instead, the feed stays ordered based on the time the posts came into the feed, with the latest post at the top. Sorting by Recent activity will continue to be the default option for community feeds for now.
To sort a feed in Yammer, select the dropdown menu to the top right of your feed.
Select the option that you want.