I think there's a difference between spanking porn and spanking videos, and I'm aware that might sound a bit pretentious and exclusionary, so let me explain! In my mind, spanking porn is videos (or photos/stories/comics/whatever) with spanking that can appeal to mostly sex-oriented people, or to the broader BDSM community - e.g. a naked girl gets spanked and then fucked from behind. In such a video, tattoos wouldn't seem anachronistic. Spanking videos are videos designed for an almost purely spanko audience - e.g. a girl dressed in a semi-realistic school uniform is given a bare bottom spanking and then stands in the corner. This type of video would probably be deeply boring to most sex-oriented people, and to a lot of the broader BDSM community, but to lots of us it'd be lovely. Obviously there's no hard and fast line, lots of videos could be argued for both categories, and it's just a personal opinion of mine.
In terms of realism...it's suspension of disbelief, innit? Like, when we watch a historical mainstream film, we know it's not magical CCTV from the Victorian era - but the costume, set, speech etc is convincing enough that we can suspend disbelief. If someone rocked up in a film set in the Victorian era wearing a latex bodysuit and a gimp mask it'd become impossible to suspend disbelief, and that would make the film a lot less immersive and enjoyable for many people. In spanking videos, tattoos can be that 'thing' that makes the film cross the line and means disbelief can't be suspended. They can remove the audience from the video and emphasise that it's just pretend.
That's just my tuppence - I'm aware many may disagree
