chanko!
Are you just making a comment, or are you looking for help? If you are just making a comment, then you have made it- "you don't like them" - and that is the end of it, but if you are looking for help, you are going to have to help us help you by giving us better information.
The Casio CTK-2500, CTK-2550, and CKT-3500 were designed specifically to work with the Casio Chordana Play App for "smart" devices: tablets and phones: Apple iOS and Android. When you say "trasfer", I assume you are loading them into the CTK-3500's internal memory with the Chordana Play "Transfer Mode".
When you say they sound bad, do you mean the sound quality is bad (noise, hiss, distortion, mushy, break-ups), or that they sound "tinny" (like a toy piano}, or that the songs, themselves are too basic, too simple, too plain (childish)?
If the sound quality is bad, how do they sound if you just play them with Chordana Play on your phone or tablet, instead of on the CTK-3500? How do they sound if you just "play" them to the CTK-3500 with Chordana Play, instead of transferring them to it? How do they sound compared to the CTK-3500's "built-in" songs?
Where did these MIDI files come from? Were they listed as being Standard MIDI Files (SMF)? Not all MIDI files with a ".mid" file extension are SMF's. Chordana Play and these three keyboards require MIDI files be SMF's to ensure that the instrument sounds called for by the file match the instrument sound sets of Chordana Play and the keyboards. If they do not match, they will never sound right.