Midair-Attack Damage is a weapon seal in Hyrule Warriors: Age of Calamity. It increases the damage dealt by attacks made while the player character is in the air.
According to this tweet by Swiffy22, there does not appear to be any forced cap to the damage buff that can be obtained from the seal. For instance, if Revali's Great Eagle Bow is built with four stacks of Midair-Attack Damage++ at a matched-seal buff of 22% each and its level 25 hidden seal at a matched-seal buff of 24%, the full 112% damage buff will be applied.
Note: A weapon receives a +5 damage buff from having any two weapon seals of the same suit, a +15 damage buff from having any four seals of the same suit, and a total +20 damage buff for having stacks of four seals in one suit and two seals in a different suit. Six seals of the same suit will only give a +15 damage buff. Therefore, if you wish to plan your weapon build in advance, it is important to know the hidden seals of your chosen weapon.