I mistakenly made a meat pizza with dough that was dairy (it had dry powdered milk) in my oven.
Do I need to kasher my oven?
Yes you do. The hot meat absorbed the taste of the milk and gave off trief steam to the oven. If you have a self cleaning oven run it on the self cleaning cycle, and your good to go. If it isn’t self cleaning, and it won’t be a libin chamur, than wait 24 hours from when it became trief, clean it out well with oven cleaner, and put it to the highest temperature for half an hour.
As a side point there there are three distinct prohibitions regarding mixing milk and meat. 1. The actual cooking (roasting or baking) of the two together. 2. If they were cooked etc. together we may not eat them. 3. If they were cooked etc. we may not benefit from the mixture, i.e. give it to a gentile or an animal. Since this instance is considered mixing them on a d’orayso level ( according to a number of poskim), there is a need for you to do teshuva, for baking meat and milk together. Additionally, you would have to discard the trief pizza in a way that no one will benefit from it.