I know this is in the competitive play section and the title is about offline play but hear me out.

Due to input/all around lag of online mode and bad CPU habits would practicing combos and such on CPUs actually make you worse at fighting real people? Or at least you can't improve your play against people this way? Given you already have your character mostly figured out by fighting CPUs.

It wouldn't necessarily make you worse against people, but it would stop making you better overall pretty quickly. CPUs are decent practice but aren't exactly high-level training.

CPUs have set in stone combos that they perform on anyone, and don't reallly extend much from there. As a result, you are punished far less for screwing up; you can actually survive a grab as Falco for example. Also, they are horrible at using defensive options, and are easy as hell to combo due to awful if any DI and don't adapt to spam or punish you for it.

Also you ahve dumb stuff like Fox trying to sweetspot the ledge from above, or anything on Metal Cavern.

An individual that solely fights against CPUs may have difficulty winning matches against other human players.
This is due to the player being so used to the CPU's movements and strategies so fighting an actual individual may prove to difficult, as the individual facing CPUs all the time will have a hard time predicting their opponent's movements and finding openings for them to attack.

Bowler (an SSF2 expert and content creator) explains this excellently well in this video.

Also, I believe a thread like this would fit quite well in the general section of Super Smash Flash 2 Beta - https://forums.mcleodgaming.com/viewforum.php?f=117

Hope my explanation and Bowler's video answered your question and concerns!

But if an individual solely fights against CPUs it won't matter if they can beat humans because they never fight them.

Yes I've noticed how people (I don't know about other people but at least me) memorize how CPUs fight if they just fight them. The part about fighting people and how it conflicts with fighting CPUs makes sense.

I've seen this video. It didn't seem to consider people who grind CPUs but don't have bad habits or a least not obvious ones like spamming whatever they're spamming (moves or options). Other than that it's good. Input lag hurt me so much when I starting playing online and it still hurts me a lot because I don't play online enough.

Um, if you say so. I thought it would be better in this section since it's at least half based on competitive play but I guess other people would know better than me.

This had some neat stuff that I wanted to know more about. Maybe I wasn't clear enough but I was also wondering about the case where someone fights humans and CPUs in about equal parts and if the switching back and forth would hurt them.
This looks like it wouldn't.

Good to know. I've definitely noticed that comboing CPUs is so much easier than comboing people and it's also much easier to get out of bad positions when you're fighting a CPU as apposed to a human.

I believe that the CPUs are a bit bad to practice with for many reasons that have already been listed, but it'z also the constant grabbing and throwing, in my opinion. If you screw up, instead of attacking and getting you stuck in a combo, they'd grab and throw, no matter what your percent. It really sets a bad example for the competitive scene. These are the faces of my nightmares.

Use auto correct.