if "API" that you mention is related to data yes it important, but not in API model. API are something that a bit technical but I think database is more important. Database perspective help the designer understand what data that available.
The example we want to create the profile page, if we already got the data model then we can put what kind of data should show in the profile page.
@daniel_roy93 Great thoughts Daniel. Would love to have you take a look at what we are building at www.conektto.io - helping Product Managers design their API by connecting data source and not having to write a single line of JSON nor requiring PhD in RestAPI standards.