Can doing squats make your butt hard since you're developing muscles there
Yes, with an asterisk.
Sure, since muscle is more dense than fat, increasing muscle mass (glutes) and decreasing fat in your butt will serve to harden it.
I feel a need to point out that while squatting activates the glutes, there are exercises that do more for them than a regular squat, such as the deadlift. So if one of your goals is to get a firmer butt, I would suggest focusing more on deadlifting. The squat is still a great exercise though, and shouldn't be forgotten either.