It is basically that easy. Choose one of the Alfresco Remote APIs and call them from your app. The available ones are
CMIS, the Alfresco SOAP webservices and the standard REST API webservices. With CMIS you have the option to use SOAP or REST with atompub XML. The standard REST API mostly works with JSON as a data exchange format. Another way is exactly what you considered, building a REST API on your own with webscripts (and JSON).
Each API address different integration aspects and covers different areas.
For mobile apps I would recommend to consider the dependencies each remote API would need. SOAP needs a lot of libraries, atompub as well…. so your app might get rather big.
In my opinion using REST with JSON is the most lightweight way. You just need a httpclient and a JSON parser. Use the standard REST API and add uncovered, required functionality with your own webscripts. I used this approach successfully.
Regards,
lothar