Playing high-level CPUs too much makes your tech skill worse, because they react strongly to some things yet never do anything against others. For example, if you try shffl dair on a high-level CPU Bowser, it will always utilt if it's not breathing fire. If you double jump dair instead the Bowser won't do anything to defend itself. Then you find yourself in the habit of always double jumping to land a dair. Or if you're off the stage and you recover low they edgeguard really viciously but if you recover high they just stand there and let you on the stage. So then you get in the habit of recovering high (I fell victim to this once and didn't even notice that I was doing it until my friend pointed it out). You might forget to wavedash after a while because CPUs don't react to it any more than dash dancing.
CPUs never do a lot of stuff that is second nature to human players. Jiggs never uses rest, Luigi never uses up-b to recover, Marth never uses side-b to recover, none of the CPUs ever run, etc. And they always approach with the same moves: Bowser always takes a few steps and then breathes fire, Kirby always takes a few steps and does final cutter, Falco always takes a few steps and shoots a grounded laser, Jigglypuff always walks toward you and tries to grab you, Marth always walks toward you and jabs you (and grabs you if you try to shield the jab).
The most important thing is that CPUs never learn, so you'll get badly owned if you practice with them and then play anyone that utilizes all of a character's moves and learns quickly. The only reason people play high level CPUs is because they want to practice but they have no friends that play the game, can't travel to tournaments or smashfests, and can't/aren't willing to play SSBM online with an emulator.
Mind you, lv. 1 CPUs are great for just practicing combos with aside from the fact that they don't DI all the time, but it gets incredibly boring once your tech skill is at a reasonable level.
But I have nobody to practice with...
