Vehicle mass is a significant factor in collisions between vehicles because taking two thirds of the impact is a meaningful 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.