Yeah, the thought of everyone becoming idle in feudal society is kinda unlikely seeing how most of those people are entrepreneurs selling their works through their crafts to make money. They don't have pensions and stocks or anything here to actually coast on. Money is made off of what they produce. The concept should pan out as long as everyone isn't sitting around with an economist on how they can cruise through the situation in the most optimal way. For people who live off their craft, lower taxes means you can improve your tools + way of life. Which is why lowering for merchants is important as they can bring both of those.
I'd say the real problematic situation is what happens if the other lords, upon seeing that the lower taxes have been pulling their merchants away, decide to lower taxes too. She presents it as a short-term solution because it can definitely help in the immediate moment. But if they don't raise taxes back to the norm, the lords as a whole can find themselves in a race to the bottom which would hurt overall finances in the long term.
But really, all of this is just speculative and any number of things can happen. That's why economists make money. They know a lot more than the random person in a comment section of a manga website.