there are so many amazing mixes on youtube and on soundcloud. many of these short ones have been finding their way onto spotify which is great. When the ben pearce remix of C O O L by Le Youth came on spotify i was so surprised!
Unfortunately, we can't just add mixtapes to the Spotify catalogue. Have a look at this topic why it is hard to add them. Also, please don't get the status "Case Closed" wrong. It doesn't mean this will never be implemented (there are already mixes available on the platform), but it's up to the artists as stated in the mentioned topic.