It's not grammaticallycorrect unless you put in the word 'much'.

It's incorrect. The party was SUCH fun. The party was SO funny. SUCH + noun, SO + adjective / adverb.

SO FUN / SO MUCH FUN Strictly a young person’s usage: “That party was so fun!” If you don’t want to be perceived as a gum-chewing airhead, say “so much fun.”