We are developing Native iOS app which is using Salesforce SDK.
Is there a way to fetch custom labels from Salesforce using SDK ? (like using RESTapi)
Not directly, no. The mobile sdk is missing the necessary functions to perform this retrieval of data. The reason why, is because custom labels are of available in the rest API. You would need access to the metadata API, which from looking at the source, doesn't appear to be available. As a workaround, you could create a custom rest API call to retrieve the labels via Apex Code.