Yes , it is the same for cover art. for some reason you are able to see one example only but you are unlikely to improve on that.
There is a a difference between a media player such as JRiver and Mosaic. The media player on your computer navigates the storage and actually plays the tracks out from the computer to the dCS DAC via USB. The Mosaic app on an Android or iOS device offers control only allowing navigation. The app does not itself play the track but instead sends a request to a UPnP server device like a NAS to send the track data to the second, hardware, part of Mosaic which is in the dCS unit. This plays the track. Instead of using a media player, the NAS has UPnP software installed to carry out these instructions from the app and the data is sent preferably via the network. This method offers optimum sound quality and you will see much more about the track title ( a ticker tape type display opens at the screen bottom which shows long titles) plus other information such as the cover artwork.
Mosaic is designed to work best with UPnP server software called MinimServer 2 and custom versions are available for various NAS brands. JRiver does have a UPnP function but I have not read great reviews of this. I have no detailed knowledge of Audirvana. Another option is Roon which you might like to google for further information. As you are already setting up a NAS then I strongly recommend the MinimServer2 route as this is what Mosaic was designed for.