Whoops! You created a viz with a custom font, installed that custom font on Tableau Server, but Server is rendering the viz with a generic parent font replacement – not your wonderful update to Comic Sans. 

Why? One of these two reasons is correct. You figure out which: 

• Tableau Server knows that the font that you chose is really, really ugly and we simply refuse to render it in order to spare you from embarrassment in front of the whole world. We look out for our customers like that.
• Tableau Server generally defaults to client-side rendering these days. So unless you force server-side rendering, it doesn’t matter that the font is installed on the Tableau Server box – your browser will go hunting locally for that font, and it probably doesn’t exist on your machine. To force server side rendering, use ?:render=false on your request for the viz. More info here: http://onlinehelp.tableausoftware.com/v8.1/server/en-us/browser_rendering.htm