Is standing in the face of those in charge of a mosque who wish to appoint an unqualified imam, or leaving the mosque and going to another, considered sedition?

Assigning the imamate to those who are not qualified for it is a reprehensible act and a betrayal of trust. If this is happening in a Western country where there is prevalent religious ignorance, then changing it depends on one's ability and on ensuring that no greater harm results. If you have the ability to replace the current Imam with another who is religiously qualified, without causing greater harm, then confront those in charge of the mosque after offering them advice. However, if you believe that a greater harm, such as conflict or the closure of the mosque, might result, then do not engage in confrontation. Pray behind the Imam if his recitation makes the prayer valid; otherwise, pray in another mosque.