The absolute worst part of playing on your own is that you get into the habit of looking at your own character too much and end up going into auto-pilot mode, where you don't actively think as you play and instead do mindless combos on a level 3 CPU that you probably won't implement into your game. You should definitely try to find real opponents who can beat the bad habits out of you until you stop them completely, esp. before expecting results at tourneys. You also can't practice the neutral game or spacing against a CPU, so often that is an area that is extremely lacking when you first play real people. For that, you should study how other high level Yoshis play, look at how they space, and pick out the different kinds of playstyles they may have that you can take from and add to your repertoire.