Is it a good idea to mention previous rejection and subsequent improvement in motivation letter? I am applying to a Master's degree in Finland. Last year I applied to the same programme and unfortunately I was not accepted.

While I am writing my motivation letter I wonder if it would be a good idea to mention that I was not accepted last year but now I have improved as a professional during last year, so I can tell that I am a better prospect now.
If you think you might know the reason for rejection and over the course of last year you have managed to resolve the issue then IMO it is worth mentioning. On the other hand if you added so much prestige to your profile during the time then it's worth stating that you had applied last year and also state your qualifications and skills earned since then.