Is the blackfaced actor used to make fun of black people? Probably racist. But that would also be the case if it were a black actor doing the same (it might just be more difficult to hire one for that).

Is it used to create tension between the obviously fake costume and the effort put into pretending that it is real? Probably not racist. I can imagine a show deriving great hilarity from casting everyone in a role they are not the least bit suited to and then trying to make it work anyway.