Vehicle mass is a significant factor in collisions between vehicles because taking two thirds of the impact is a major increase over taking half of it. Heavier vehicles are much more dangerous to lighter vehicles.

It's basically irrelevant in pedestrian collisions because the pedestrian is already taking ~95% of the impact even from a lighter car and the <5% difference between that and an arbitrarily heavier vehicle is completely dwarfed by any of the other variables like the speed of the collision. For a pedestrian it's better to get hit by a freight train at 35 MPH than a 3000 pound vehicle at 39 MPH.

I was thinking of F = MA, where mass seems important here. Is this the wrong mental model?

You're solving for force when the thing that causes the damage is (very rapid) acceleration. The total acceleration is the initial difference in speed between the two things that collide and the proportion of the acceleration applied to each object is inversely proportional to their relative masses.

If they have the same mass then they each take half. If one of them is 19 times heavier then the other one takes 95% of the acceleration. If one of them is six hundred trillion times heavier then the other one takes ~100% of the acceleration, but this is still only <5% more than taking 95%.

[delayed]

You think cutting the weight of the truck is going to make it safer for pedestrians?

This is the silliest argument against the Cybertruck, IMO. Its forward visibility is significantly better than other full-sized trucks, yet people continue to bring up pedestrian safety every time the Cybertruck is mentioned.

Sure, I meant htis for electric trucks in general, and I was wrong about the physics anyway.