Is this in praise of something someone did?

Nope, more like a compliment

YOU ARE JUST FABULOUS sounds highly unnatural, and rather affected. If someone said that to me, I’d think they were joking or being sarcastic. Especially using YOU ARE separately.

It sounds like a caricature of a very camp gay man addressing someone.

https://memegenerator.net/instance/64831570/gay-richard-simmons-hey-jayme-youre-just-fabulous-you-hunka-man

Евгений, are there not enough translations to Russian on this site that you have to keep adding English translations that require correction?

https://www.google.com/search?q=%22you%27re+just+fabulous%22+meme&sxsrf=ALeKk01xLSXkBr-uPmCFU41KFRKImy8uPw:1599236595115&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiAhafJ9M_rAhVDRa0KHSHLCGIQ_AUoAXoECAsQAw&biw=1503&bih=882#imgrc=pgk_yta6X_YRjM

http://memegenerator.net/instance/29166486/big-gay-al-oh-honey-you-are-just-fabulous

Is the original Russian campy too, or can it be sincere?

Nope, more like a compliment

In that case, I would use AWESOME. Someone might actually think you were being sarcastic if you used FABULOUS.

And definitely YOU’RE, not YOU ARE.

Why not translate it directly - "You're a / my fairy tale"?

...or you're fantastic

It sounds too disingenuous. We don’t call people fairy tales.

FANTASTIC is almost ok, but, again, borders on sarcasm. AMAZING would be ok.

Uly,
Please don’t think I’m arguing, I just want to understand why an expression that’s unusual and isn’t idiomatic can’t sound fresh and vivid in English as well as in Russian? For example, when my wife and I just got married, she often called me “ты мой демпферочек” meaning that when she got worried, I was like a damper to absorb her worries. I think that “ты - сказка”, not being idiomatic, is much easier to understand although we also don’t call people fairy tales.

There are only certain things you can call people when you’re being sincere and others when you’re being humorous or sarcastic. For instance, saying YOU’RE SO INTELLIGENT is a sincere compliment, however YOU’RE SO SMART is the choice when we’re being sarcastic and actually mean “You think you know everything, don’t you?” That’s why I say context is everything - but also usage. When and how natives use certain word depends on actual usage and that’s something a native knows instinctively and it varies from word to word. For example, the word FABULOUS and especially JUST FABULOUS is something a (usually gay) man says to another gay man or to a drag queen who looks unbelievable. You’ll never hear a straight man telling another person that THEY ARE JUST FABULOUS unless he’s absolutely taking the piss out of someone (see the memes I posted above). So to answer your question, this isn’t so much a linguistic questions as it is a cultural one and I don’t have a ready answer.

The way I see it, calling our SO non-idiomatic names at one point or another is quite common in any culture. But using something that has "history" we both know and feel is one thing, quite another thing is using a non-idiomatic name to address someone that might not click with them.

Absofuckinlutely! Tatiana, I couldn’t have said it better myself! Brava!

😌🙏

On the back of Tatiana’s wonderful explanation, I’ll say that I have no idea how or when FABULOUS became a campy word in reference to a person, it just immediately conjures up a certain type of speaker in my mind and that’s not linguistic, it’s cultural.

👍

👍🙏