The phrase 'it's so cute' is correct and usable in written English.
You can use it to express admiration or enthusiasm. For example, "My niece just learned how to do a cartwheel - it's so cute!".
Their refrain matched what I hear when I'm forced to take my friend's Shih Tzu for a walk: "Oh, it's so cute!" As with that little dog, I heard it enough times that I began to believe it myself.
I can't tell you how many videos I watch online where people think it's so cute that there's a child climbing onto a dog's bed and making erratic movement and squealing, not respecting the dog's space at all.
It's a minitrain, it's so cute".
"My wife is surrounded by superhero geekiness but is actually delighted with Millie's first word as it is so cute," he said.
The composer Thomas Newman, who scored Stanton's earlier films, told me, "On 'Nemo,' I said it was so cute and funny when Dory" — the blue-tang fish voiced by Ellen DeGeneres — "is talking whale talk.
They thought it was so cute.
