Well the specific problem you have here, i would say that is an issue. They might not focus on it if it is non reproduceable or fixed by refreshing. But that is for the devs to triage.
Some floss projects have a dicussion forum to make general suggestions or as questions. I imagibe there is such a thing here on lemmy if not on github.
But ui suggestions can be issues. For example some reddit ppl (like me) have been confused because the add comment box is at the bottom of the thread instead of the top. I think that would be a fair issue to open. Or if you think the top menu bar should be arranged differently. Or if the colors are unreadable, etc.
Projects that only want certain kinds of issues reported, they should create a document explaining this.
An awful lot of discussion about what to replace reddit with has mentioned “the UI” as a major barrier to this so I hope the devs would be receptive to feedback. But they dont have to and shouldnt take every singke suggestion because it would create a “too many cooks in the kitchen” situation.
the admins should put a link at the top to a search or version of the feed or something that filters out all posts with “reddit” in the title or body or linked article to show what the site is like when it isn’t consumed with gossip and drama.