Its a bit crazy, in the sense that the same mechanism for dealing damage to other mechs (how big you are and how hard you can kick) determines how many PBIs you get rather than how many you can catch (I would tend to think stomping should favor faster mechs over slower ones), but in general each stomp is only going to net one or two PBIs, which is pretty reasonable, and that's when you connect. Infantry are trained to avoid mechs (unlike the poor suckers in the MW games), spread out quite a bit, actively shooting back (which may or may not worry a given mech), and in non-open terrain they're dug in and out of sight. Its quite reasonable for the attack to be tricky and low gain.