More precisely, the definition of media type includes the parameters, and the following is thus possible:
 <link rel="stylesheet" href="styles.css" type="text/css; charset=utf-8"/>

(even though the charset attribute would be a better idea in that case)

The mobileOK Checker only matches stylesheets when the type attribute contains "text/css" and nothing else. The above stylesheet would be ignored, meaning that it would not be taken into account for page size and that no tests would be run on the stylesheet.