Google Webmaster Pagespeed recommends to inline CSS files (see developers.google.com/speed/docs/insights/OptimizeCSSDelivery) to speed up page loading, especially for mobile devices.
As far as I have seen in Yii 2.0.3, you cannot set such in-lining in AssetManager or in the registerCss method.
Nevertheless it would be nice to have this feature for production sites.