My wife just came to the US in August 2018 when she became a resident (from abroad). we are married filing jointly, does she need to file part-time NYS taxes?

If your wife has no income from any other state besides New York, you can file Jointly a New York Resident return, even though she didn't live all year in New York, as she has no income to allocate to another state. 

Here's more detailed info:

https://ttlc.intuit.com/replies/4777207