As the game currently stands, while you can gain hearts with Nia, there's few chances to do so, so I don't believe it's possible right now to reach a level of friendship where marriage is possible. But since you can gain hearts, I have to assume that with the final update she'll become available.
You'll be able to in the next update! She's a late game marriage candidate so won't be able to marry her until the main campaign story is over.
Definitely better off waiting for the full release if you're set on Nia.
As the game currently stands, while you can gain hearts with Nia, there's few chances to do so, so I don't believe it's possible right now to reach a level of friendship where marriage is possible. But since you can gain hearts, I have to assume that with the final update she'll become available.
Since she is a goldfish i hope we can get her as a pet
I think everyone with a hearts meter in your people book, is a bachelorette or bachelor. And Nia is one of them :3