Silly? Absolutely, but it’s also clever, amusing, and deep.
Why deep? Lego Batman is successful and proud, but isolated, lonely, and feeling more and more rejected as he realizes that his time has passed. It’s no longer cool to be a vigilante; justice has to be dispensed democratically. Lone heroes are old hat; today’s heroes need to leverage teamwork. No man is an island, not even Lego Batman, who lives on one.
Seriously, this cartoon beats Dawn of Justice hands down.