Interesting question. Even though, 'fun' is doing the job of an adjective here, structurally it's still a noun - you can tell by the fact that it's modified by 'so much'. Only a very small child, who is still experimenting with grammar, would assume 'fun' was a true adjective and say 'It's very fun' or 'It's funner'. 'Fun' is actually a noun, and is always modified as a noun. That said, please don't accept this as 'good grammar' or a construction that you can extend to all nouns. In very informal language, particularly the speech of younger people, there are a handful of nouns which are sometimes used as adjectives while still retaining noun status. 'Fun' is a common one, and there are a few others. For example, a youngster might say 'That was a rubbish book', meaning 'poor quality'. Similarly - and even more informally - the word 'sh*t' can be used in this way. For example, it's not unusual to hear someone say 'I had such a sh*t time.' Sorry to give a vulgar example, but this is common, and also quite interesting linguistically.

I know this question is resolved, but I thought I'd add something for you. - I am tall. - Dogs are fun. The verb "to be" is called a linking verb. When both sides of the verb have nouns, they are equal in identify. I am a boy. He is a man. She is a girl. They are soldiers (they = soldiers). When an adjective is on one side, it is called a predicate adjective and describes the subject. He is tall. He is a man. He is a tall man. He = tall man. Dogs are fun. They are fun dogs. They are dogs that are fun (to play with). In "Dogs are fun", "fun" is a predicate adjective that describes the subject.

Adjective