請問戴印有虛構警徽的帽子犯法嗎？

It really depends on the design. Generally, the answer is no, as soon as the appearance will not make anyone confused that the wearer is a police personnel in the jurisdiction.

For example, if you wear a "FBI" jacket in the U.S., that can be illegal. However, it will not be illegal elsewhere.

雖然刑法第159條規定：公然冒用公務員服飾、徽章或官銜者，處五百元以下罰金。但只是一頂帽子，應沒有問題。