That's because she has a hidden additive damage increase as-is. From the wiki:
Peacemaker has a built-in damage buff, increasing its base damage by 100% / 115% / 133% / 150%.
This buff is affected by Ability Strength and stacks additively with Damage mods, such as Hornet Strike.
Thus, if you have let's say 154% ability strength (Intensify + Augur Secrets), you have an extra 1.5 * 1.54 = 2.31 modifier in the formula - hilariously, like having a second Hornet Strike equipped.
Going from +451% damage (Hornet Strike + 54% ability strength Mesa) to +616% damage (Hornet + Magnum + Peacemaker bonus) is only a gain of ~36.6%.
That's why it feels so weak on her, and why it's pointless to use (and why Anemic Agility is so good on her, while we're discussing). Put it on your Catchmoon instead. 😛