This is probably a separate issue, but one thing that stands out to me when comparing these Slack and Mattermost screenshots is how the users' names are much more distinguishable due to color in Slack than in Mattermost. I don't know why that is, or if it's easily configurable in Mattermost, but increased color variation in the names could be quite nice on the eyes.
I suppose this is at least somewhat related to the compactness feature, because as more conversation is displayed at the same time, it becomes more important to be able to quickly distinguish who is saying what.