While they can both be used for dashboard, they seem to be quite different. From what I can tell, Superset is a platform while Tabler is a html/css framework. If I'm understanding it correctly... Superset integrates with a database, but as a framework Tabler needs some sort of backend to drive any database interaction. Superset has it's own UI that you interact with while building your dashboard, but Tabler is just a set of elements that you'd need to use to build a dashboard using whatever web development methods you prefer.

So they're for different purposes even though they both result in dashboards.