The key is in the presence of the word "mandate".

Authoritarianism is a form of government characterized by highly concentrated power, limited political pluralism, and the suppression of dissent, often enforced by a charismatic leader or elite group .

A mandate is an authoritative command, order, or authorization to act, typically given by a higher authority, such as voters, a court, or a governing body .

So in the sense that a mandate is passed by government, and governments are sometimes authoritarian? If your logic is stronger than that you'll need to explain it to me. I'm not saying Asian countries are not authoritarian, I take no stance on that, I just genuinely don't understand how mandates imply authoritarianism.

"Mandate" on publicly owned and constructed parking lots?

That is, parking lots paid for by government taxes?

Why is this mandate a bad idea?

Yes, it is.

Why cannot mandate what they want on their own parking lots? This is not about private properties.

The sensibility of a policy and the power dynamics of it’s application are orthogonal.