I go back and forth on this one. Unlike the names Tiana, Aurora, and Arielle, I have never heard the name on real person. But unlike Cinderella, Rapunzel, Mulan, etc...Merida seems much tamer and more usable, and as far as I know doesn't have any major folk-lore or historical connections. I would say that in a few years, it will be usable - but people will probably associate it with the princess and make comments here and there, but I wouldn't expect a lot of teasing or eye rolls from it.
I don't think the tie is that strong actually. I have actually asked people this, and they didn't remember the characters name was Merida. It not the biggest Disney movie, and I think the ties are very weak. Especially in 5-10 years.
This in my top name lists, and I would definitely consider it usable.
I have not seen the disney movie so I didn't even make the connection.
I did however make the connection to the city in Mexico! And it was a city name before a disney character name!
I think it is TOTALLY usable! cute too!
The names that have sprung up from Disney movies often have to due with current name trends and how common the name currently is.
Jasmine was already in the top 50 when Aladdin released. It's longevity was probably due to the movie, but the connection was mostly negated. Ariel still has stronger ties with The Little Mermaid, but while it definitely peaked when it was released, Ariel had a history as a given name in America.
On the other hand, Belle still has stayed off the list. But the name is associated with so many other things (as a nickname, Isabella and the other rising -belle/-bella names, southern beauty, etc.) than just the movie.
Anyhow, I do think Merida will be useable in a few years. It's not the kind of Disney movie that will be considered a classic IMO (i.e. Nala and Nemo are so associated with Disney it would be hard to pull either of them off), and even now people are forgetting the main character's name. In seven years, some may think when they see a little girl Merida, "Wasn't that the character of a Disney Movie? What a great namesake!" But it's definitely not like Cinderella or Mulan.