Isn't that multiplier one of the last things applied though? Would it actually affect the armor for a single hit?
State Moves, Deep Breathing, Monado, Psych Up, Oomp, SH aerial penalty, ect. are taken into account before armor.
1v1 multiplier is
not, and is the very last thing applied. A singular hitbox will give the same outcome in 1v1 or otherwise. However, cumulated damage from
previous hits will be reflecting the total, multiplied damage.
Mario Jab does (base) 2.2%/1.7%/4.0%. Without taking into account stale moves, that does 7.9% total. This is not enough to break 8% armor, such as on Mac neutral-b.
However, in 1v1 the cumulative damage applied to the first 2 hits would be counted, as 2.64%/2.04%. While the armor check on the last hit would still only reflect 4.0% at the time of the check, it would consider Mac having taken 4.68% + 4.0% = 8.68% total, or enough to easily break the armor.
In other words, outside of 1v1, Mario can only break 8% armor with a fully fresh jab combo. In a 1v1, he can break 8% armor with a slightly stale jab combo, as long as it wasn't the last 2 moves used.
Edit: This is a great time to advocate my claim that a small armor buff to Straight Lunge is one of the things that would make the most difference for Mac. (Exactly like K. Rool and now Bowser Jr. got, except their armor values started lower and needed a bigger increase than Mac would require.)
Currently, Mac can use his 8% armor Straight Lunge as an alternative to air dodging, thanks to the freedom to shield cancel it as a compelling alternative to an air-dodge's ending lag. (The benefits being frame 1 + armored landing, at the cost of different commitment.) However, 8% is a very limiting application--you can only get through say, a Mario uair or Pikachu uair--and not one of their other aerials. A defensive option that only works on one move is... just dubious.
But at around 10.5% (What Bowser Jr. got), Mac jumps to being able to armor through typical speedster aerials like Mario nair, Pikachu nair, Pikachu bair, Wolf fair, non-Arsene Joker bair, or even Mario bair if it's not completely fresh.
That's a
vastly superior landing option, even if it's vulnerable to plenty of counterplay. But Mac can respond to those answers with his own, other tools, and the final picture is much more rosy for him.