[yedava]

I don't think 'mobility' is a factor for squats. If you think you have bad mobility, try the low bar squat where the back is more horizontal at the bottom of the squat. I have a great deal of difficulty squatting with a vertical back, but I find squatting with a more horizontal back quite natural and easy.

[vidarh]

Yes, it very much is an issue with squats for many people.

If your posterior chain is too tight, you will round your back to compensate for an inability to hold your legs in the right position while hinging your hip enough.

Low bar if anything requires you to be able to keep your back tight while hinging more at the hip.

A lot of people can't do that without a few weeks of stretches first, thanks to years at a desk.