The Puddlestomper's Guide to the Tri-Matron System

Let me first apologize for the wall of text, my critiques got away from me, lol, but I wanted to be thorough.

Okay, I will concede I forgot how abysmal the stats are on the base Atlas already, but regrettably, the buffs you added are not really enough to get the Atlas into the realm of being on par with the other mechs numerically. 

To the meat of your point, in my experience, stray 2-3 damage hits are... not very common. Most tier 1 enemies do 4+ damage on their attacks, and it only gets worse as you go up in tiers. Most of the particularly common strikers are dealing 5-6 damage, and sure, you're not trying to face tank a Demolisher, but you can't even handle an Assault realistically. Given you double damage from exposed before applying armor,  most enemies are still doing 7 damage minimum to you from that. And that's just going off the average enemy, because realistically, you're only ever applying this in Melee, so let's look at the melee enemies at tier 1.

The Assassin does 4 damage, doubled if you're prone, immobilized, shredded, or stunned. Being generous that it hasn't successfully tripped you, it's doing 7 damage to you, 15 if it has. The Assault's combat knife has 4 damage, and that's not even its good attack. The Bastion only has 3 damage on its melee sidearm, so I'll give you that one. The Berserker is dealing 7, and reasonably likely shredding you, so 13-14. Obviously, you discounted the Demolisher, but that's a hefty 19. The Ronin deals 6 base damage, +1d6 on crits, so looking at 11-23 damage. The Sentinel has 4 damage.  The Specter starts at 5 damage, doubled if you're alone, which you're likely to be as you're faster than the average mech, so that's 9. Averaged together, the MINIMUM damage of enemies at this tier is 5.375, not including damage traits, so over 10 with the double from exposed. And that's just the melee options, many of the ranged options lean much more deadly, but I'll concede that you're not likely to activate it if you're not in range. In tier 1, if you only put points in hull, at most, you're getting 22 HP with full Hull investment, grit, and Personalizations, that's not a small amount of HP, but it's not exactly Take 7 to the face, and then also survive two more enemy attacks. And it's only getting worse from there because you've hit the ceiling, you're only gaining 1 more HP every other LL, while enemies are doing 2-4 damage more per tier. Admittedly, several enemies do Multi-attacks rather than getting more damage on their main attacks, but that means that enemies that were dealing 5 damage, are now doing 10 damage, which you just can't keep up with largely. Basically, it is only really useful at shrugging off attacks if the enemy team is blind, which like... I admit my table might be a bit more War-gamey than the average table, but it's a pretty common sentiment I've also heard that your GM should be treating their NPCs as being capable of like, actually strategizing and adapting to your mechs, you're not gonna be targeted with the little damage, you're gonna be hit with the Big Damage Buttons, which are also way more common than the little damage options. Also sure, your allies can theoretically ping you to proc the reaction too, which I'll concede is less atrocious for your survival, but then. Your allies are trading your attack for theirs, and odds are good, they can do more with their turn than you can. 

The Passive Core system reads like something that should be trading HP for damage, but you are realistically speaking not maximizing the difference in HP effectively here. Most skirmishes with a Main/Aux are dealing on average 5.5 damage, and that's not worth taking an extra 6 damage to apply usually. Keep in mind, the Atlas gets a free 1d6 damage a round against prone targets, AND can reduce damage instead of taking more, AND jump away to avoid multiattacks, and it's still widely considered made of tissue paper and one of the weakest mechs in the official release. Regaining HP is nice, don't get me wrong, and it's a very potent heal, but it also only has 2 base repair cap, and you have to choose between regaining HP and spending one of your 2-5 Repairs (which unlike your HP, will never get higher than 5 after you max out Hull), or not doing that and regaining one of your precious repairs, which will leave you very likely to get hit by whoever's next, so realistically, you're healing every other turn, especially in longer missions. Not to mention, you're going to be burning those repairs rapidly repairing the systems and weapons you blow off when you structure, which will be... frequently. If two enemies jump you at once, you're taking on average 10 damage, 15 with the core passive, which basically forces you to always be burning repairs to heal, and heaven help you if you fail to crit.  And not being able to spend repairs during a rest to heal often means you'll be at less than full before your turn,which only means you are required to go first to fish for a heal, and that leaves you exposed for all the enemies to shoot you, rather than the allies that are out of their range. I think you end up having a banger first combat with this mech. Your second is considerably worse. Your third is a death spiral.