It's not so much mistakes, though that does come into it, it's things like positioning, speed of recovery, physical attributes, the ability to dominate clever opponents, success in the tackle, ball-retention under pressure etc. It's worth remembering that Walker was once much maligned on here by plenty including myself for his susceptibility to gaffes but over his last couple of seasons with us he made big improvements on that front, same for Davies this year, so given regular playing time no reason why Trippier cannot similar close the gap.
I count myself among Tripp's biggest champions but mainly for his exceptional attacking prowess. Suppose we all see things differently and you are as much entitled to your view as anyone else.