Most of the time it's used as an offensive comment. But if I'm joking around with a group of friends I could say, "ah, man, sucks to be you!" and it would be funny. But those are for lighthearted situations. For more serious situations I would not use that expression. It's slang so the meaning can be vague at times and is heavily dependent on the context of the situation.

It is offensive when you have given no context. It is usually offensive and as a learner new to the language you should always refrain from using copying or trying to imitate it. However there are contexts when it could be said sympathetically. Such as when somebody is very depressed or seriously ill, a good friend at the right time in the right situation could express their sympathy by saying "It sucks to be you" But it is unlikely they would phrase it so. "what you are going through must really suck, I do feel so sorry for you" or a similar sentence would be more appropriate if you really want to use the word "sucks" Most commonly for expressing sorrow or sympathy use "oh that sucks" combined with the correct sympathetic tone manner and body language.

Instead of saying "It sucks to be you", you can say something like "That sucks" or "That's not good/nice".

(Part 2) There is a famous example of how the same phrase can be either a joke or an insult in Owen Wister's 1902 novel, "The Virginian." This novel set the pattern for many "Westerns." A man from the East is visiting Wyoming for the first time. Wister blanks out the fourth word of the phrase, but I will include it, because nowadays "son-of-a-bitch" is not as bad as it was in 1902. Early in chapter 2, we read: '“I suppose you have me beat,” said Steve, grinning at him affectionately. “You're such a son-of-a-bitch when you get down to work.” I had expected that the man would be struck down. He had used to the Virginian a term of heaviest insult, I thought. I had marveled to hear it come from Steve's friendly lips. Evidently he had meant no harm by it, and evidently no offence had been taken. Used thus, this language was plainly complimentary.' So, here, it is a friendly joke. Yet, later, in the same chapter, in a tense poker game: 'It was now the Virginian's turn to bet, or leave the game, and he did not speak at once. Therefore Trampas spoke. “Your bet, you son-of-a-bitch." The Virginian's pistol came out, and his hand lay on the table, holding it unaimed. And with a voice as gentle as ever, the voice that sounded almost like a caress, but drawling a very little more than usual, so that there was almost a space between each word, he issued his orders to the man Trampas: “When you call me that, SMILE.”' The Virginian is prepared to shoot Trampas over the same phrase, which in this context is a deadly insult. (There is a tense moment as Trampas decides whether to pull his own gun, but he backs down).

(Part 1) Yes, it is offensive. Sometimes people insult a friend as a joke. A informal phrase we use for this is "just kidding." As a foreign speaker, you should not try to do this. It is dangerous. It is only safe if you use exactly the right tone of voice and body language. You also have to know your friend very well--for example, you have been playing this social game of joke-insults for years and years. Worse yet, bullies sometimes insult people, and when someone objects, they say "just kidding, can't you take a joke?" when the reality is that they were _not_ just kidding.