You can use Metadata tagging on your media files and the Roku will read a number of the tags like title, album, comment. If you use the album tag it should/will override the folder name the file is in. If the album tag is empty the Roku Media Player should use the folder name the file resides in. You can also embed an image file so it gets displayed by the Roku as a thumbnail image. Or one can use a folder.jpg file so all media in the folder uses the same thumbnail image.
There are a number of programs that can be used to modify media file metadata tags. The free MP3Tag (https://www.mp3tag.de/en/index.html) is one popular program for tagging MP3 and MP4 files. Media Monkey (https://www.mediamonkey.com/) is another free program that is popular.
Also it may depend on how you are accessing your media with the Roku Media Player channel. For example I usually select Video, then the My Cloud media server, then Video again, then Album. This way the Roku displays my media based on either the album metadata tag or by folder name if no album tag.
And this is what the folder structure looks like on the My Cloud for one particular folder. Since I’m not using the album tag the Roku pulls the folder name instead.
And MP3Tag showing what the files metadata tag looks like.