that's nice

Generally, there are two ways this phrase is used. Either it's a synonym for "Why are you telling me this?" as well as "I don't particularly care," or the person who uses it generally considers what they're responding to as, well, 𝘯𝘪𝘤𝘦. Typically, the former is implied more often than the latter. Often, especially over text, it can seem like someone is being sarcastic when they're really not, because, you know, you can't really convey tone over messages.

Why-are-you-telling-me-this-type:
"Ayo! Connor got this new game and it's totally sick!"
"That's nice."

Genuine-type:
"I feel pretty today."
"That's nice."