No, you can't say "How fun it is...".
As your post shows, "fun" is also a noun:
My friend is great fun!
We had great fun at the party.
Let's have some fun.
Why don't you stop working for five minutes! Don't you ever have
<Side discussion about whether maths is singular or plural moved here.>
Nat told me "My friend is great fun" doesn't mean "My friend is funny", and I was just wondering if "My friend is great fun" means "My friend is humorous".
But, Pob, since "funny" and "humorous" mean the same thing here, why doesn't "My friend is great fun" mean "My friend is humorous"?