I did something a bit odd when I first started playing. For myself, I simply picked up a character and played with competitive humans almost exclusively for some amount of time. It was infuriating to try to do this, but they were helpful enough to teach me what things I ought to try looking into. After that semester was done I pretty much played Melee against the CPU for 1-2 hours every few days or so doing stuff.
After returning from that summer I had a vast improvement in playing.
I think using the CPU effectively requires you to have a good idea of what competitive things to even try practicing in the first place and its hard to say anything I feel unless you had that exposure at least a little bit to other competitive players. It gives you valuable feed back about what really works and what does not against humans.
So, I would say to use the CPU effectively lv 1-3 are ideal to simply copy things you see from videos. You do not have to worry the distraction of getting beat up every few seconds because you fail. However, you sometimes want to ramp up to the lv7-9 because these CPUs will actively try to attack you. While not the same as a human, it attempts to replicate at least a little bit the distractions you will see in a match. This alteration between low and high lv CPU should help to mitigate CPU abusing habits.
Eventually, CPUs I feel are not the best practice and you need that combination of CPUs to test stuff and refine some things you lack when playing human. Though you still need that human support to give the proper feedback to use in your CPU play I feel.
What character are you even using?