If the non-hatchet 'mech is allowed to kick and the hatchet one isn't, the hatchet one would probably lose more often than 50% of the time.
I seconded to it. I don't think that the mech with hatchet can beat its twin, without very good luck.
Even for the worst situation, and even without TSM, a mech with no hatchet can dismantle the enemy leg with up to seven hits. It is perfectly guaranteed, and it is actually the worst case and usually around 5~6 kicks can end the opponent. And remember that it only assumes that there is really no other weapon at all so you have to remove entire of armor by only the physical attack.
Hatchet? For a 100 ton mech, only head has lower armor than its damage(20). Hit the head means it is killed instantly, but that's only a 2.7%. I don't think that you can hit the head during up to seven kicks its leg without a pure luck.
Only for insta-kill? The opponent also opt to punch. Two punch attacks have around 30% chance to hit the head at least once. Even without TSM you can kill the enemy with two hits, and damaged head internal also means you have the chance to destroy the cockpit system before completely destroy head internal.
With TSM? Well, then hatchet can make some damage against its twin, for only Center Torso and Legs are able to withstand 40 damage once. But meanwhile its twin neutralize the hatchet sibling with only three leg attacks - Only two hits of TSM'd kick is need to destroy a leg so it is 50% chance to only destroy it by two attacks and even if it is failed the third hit inevitably destroys the leg and neutralize the twin.
The above are only think about the face-to-face situation. Then what about the backstab? On rear side, punch attacks are ignores leg, and it have 50% chance to hit either torso(rear). With head, which is a weak spot, it means a punch attack can attack good location by 2/3, while hatchet have 1/2.
In either way, I don't think that hatchet can win. Or win with enough instance to be viable. It's no more than a handicapped match.
So, what about to have a fight with a 100 ton mech with hatchet and a 75 ton mech with two functional hands instead? It would be better, if we consider fair play.