Thanks @MichaelD, appreciate your feedback.
At the moment there isn't a way to easily add a custom font to Mattermost. However, would you like to contribute this in the feature idea forum so it can be discussed, upvoted and considered for a ticket accepting pull requests?
Please include a link back to this issue. If you're interested in implementing, please say so and we'll prioritize the review.
You get 10 votes in the feature idea forum, and each one influences the future of the project.