Should my tires look like this? 18 psi, foot braking.

Mine do the same

It’s normal to see “witness” marks . Some cars always do it , some don’t . If it bothers you call MT and ask them .

The reason I asked is that the last set didn't do it even when it was going 1.38 60'. These tires are five or six years old so I'm wondering if the sidewalls have gone away.

At your level , they should last/work with the cords showing . My buddy has a nova 9.30’s with cords showing lol

My m/t done the same thing a couple of years ago. I had a slow-motion video of my tires leaving the line. It seemed like the car was running over the tire. I put more air in the tires 19.5 psi stop making those marks on the sidewalls.

I'll experiment with more tire pressure next time, heck I was tickled it hooked the one time it did. This track isn't known as a good hooking track.

You might need to up the pressure a little, try a 1/2lb each time till the 60ft drops off. We still get the witness marks at 19psi but it seems to be the best "all round" pressure on the M/T pro radials for us.

Nothing "wrong", just add some air pressure if the track will take it.

Forgot to add that I also ran them down til the chords were showing running mid 9s and low 1.3x sixties.
When the tire was down to where the tread was gone and smooth, it was only about 1/2 used up. That was on the old style M/T ET Street radial though...not sure how the new ones fair. I think I used to get around 50 or 60 passes out of a pair.
Just got my first new set of ET Street Rs last week.

We got around 40 passes but the car is heavy and I probably overdo the burnout, the slick looked like it was showing a chord "pattern" but they were still black. This is on a Pro Drag Radial which is a full slick. Last time I got to the chords they were white / grey though that was a bias ply. No big wheelstands, 60fts were off some and we dropped a 10th in the 1/4 on the last 4 passes which was the other symptoms. Not out until Sept to find out if this was "definitely" the problem.