CPU's can be good for experimentation but they are no replacement for real people. Also, unless you're at a decent level, playing against CPUs will make you worse because you are not yet able to truly differentiate between them and real playrs. For example, I've noticed that non-experienced players who often play against the CPU have terrible habits like excessive rolling/spot dodging, trying to always do the same set ups (for example, Fox's dthrow into fair or Ness's PK fire into a fsmash), and being overly aggressive. Do Level 3's to practice spacing and get a feel for a variety of set-ups and, only when you can pinpoint your bad habits, then do Level 9's. Once again, CPUs are no substitute for the real thing and you'll only truly learn if you play a wide variety of people.