I've moved your question to the more general Tableau Forums. The original place you posted this is specific to the web data connector developer platform.
You are more likely to find a more helpful answer here.
The dashboards that you are exporting, are they of fixed width/height or is the dashboard set to size automatically?
I suspect that Tableau may export dashboards in a set resolution if the size is set to automatic.
Have you tried exporting the views from Tableau Desktop? Is the result the same?
I'm not sure what you mean by ugly. Do you have a few screenshots that could demonstrate the issue?