Recently I have been looking at my squat form and notice that I always bend forward too much such that my head is past my knees. When I try and correct myself to have head and knees aligned, it's just not possible, or I lose my balance and can't go straight up and down without bending forward. I can remedy this by propping my heels on some weights, so is hip mobility the issue here?
This is completely normal. A squat involves bending at the ankles, knees, and hips such that the torso leans forwards throughout the lower portions of the movement. The degree to which it leans forwards depends on squatting style and individual body proportions.
You are not supposed to be keeping your head and knees aligned, and you are not supposed to try to keep your torso upright.
When you squat your butt sticks out backwards, and it contains a lot of your bodyweight, so you need some weight out in front of you to counterbalance this in order to remain in balance. This is usually achieved by the shoulders moving forwards until the barbell is in front of your feet. If you try to prevent the bar from moving forwards, or prevent your torso from leaning forwards, then you will not be able to remain in balance, and you will fall backwards.
Putting your heels up on blocks is giving yourself an artificial ankle mobility boost, allowing your knees to travel further forwards, hence meaning your butt doesn't travel as far backwards, hence meaning less forward counterbalance is needed. So while this does result in a more upright squat, it should not be regarded as as "solution", because leaning forwards in the squat is not a problem.