Is the "ka" necessary? Does this this translate more directly as "Lots of fun" with fun as a noun?
With the given word order, the ka is needed. It is a complete sentence - It literally says The fun is big/great/large/plentiful.
If you want to say very fun literally, then that is leʻaleʻa loa.
duo did not accept my: leʻaleʻa loa
Why isnʻt Leʻaleʻa nui?
First, this is an exclamation and thus, it is a complete thought. Because of that the word order above is required in order to made it a complete sentence. If you want to just say the phrase very fun then le‘ale‘a nui is possible.