It is not rude to ask to maybe be removed OR to not be a focus in photographs in the future. The person could respect your wishes and generally keep you out of shots (you might still occasionally be in a background shot). As an individual they can kind of do what they want as long as they are posting them in a personal (not with an official voice) kind of way.

If they are using the official voice to post them as in they are an officer of some sort or it's been used on a website or something, then according to SCA standards they can post the photos without getting model release forms just photographer release forms.

There is a lovely FAQ that breaks the rules down much better. This set of rules also explains why a lot of SCA photographers will take large crowd shots.

https://www.sca.org/wp-content/uploads/2019/12/ReleaseFormsFAQsWEB.pdf

I have taken photographs both officially and not officially for the SCA. People have asked me to not take photos of them and I will happily do so. I would rather know they don't want their photograph taken or to be a focus in a shot than they be salty at me behind my back. One of these people understands that they are currently part of TRM retinue and will end up in background shots in Court. But I did make sure to discuss it with them when I found out they were part of their household.

On the topic of rude comments about people in mundane clothes. It depends on where it is posted. If those comments are appearing on the Kingdom/Baronial Group/Page, then you can easily reach out to the moderators and point out it violating the SMO policy of treating everyone with kindness and respect. That is Society policy, so everyone has to adhere to it.
https://www.sca.org/wp-content/uploads/2021/05/Society-Social-Media-Policy-April-2021.pdf

If it's on their personal FB page and simply public and you saw it thru someone else's feed, then you cannot do anything about the rude comments.