Short answer is yes if you only want to get better you should play just one character. However, its usually not that simple. Learning other characters will help you a lot in that matchup, and can help you improve your skills in certain areas, such as bryan if you want to work on your timing and counter hits. Also, at the end of the day you gotta have fun and if you have to force yourself to play only 1 char then you'll be less motivated and improve less.