Can't be worse than those folks posting songs with just a stock image, which regularly ticks me off when I'm looking for an actual video clip... What was the question?
Oh, right.
Yeah, I feel ya. I'm currently working on a promotional clip for my Patreon campaign, using my webcomic panels 'cause that's what I want my would-be patrons to back and there's no way I'm getting in front of a camera if I don't have to.
Legally, make sure the license for the clipart covers free use in YT videos. YT isn't non-commercial, even though its use often is, so a Creative Commons NC license might not be enough.
Technically, nothing wrong with having an animated slide show rather than actual moving pictures.
'Culturally', it depens on what graphics you use and how you use them.
Kirby Ferguson, Tony Zhou and
Adam Westbrook have done some amazing stuff with pre-existing images and footage, but in each case it
had to be that material. So there are ways to do this that aren't tacky and unprofessional.
Ask yourself, though: If you don't really have a visual message, is video really the right medium for you? Maybe you should be going for pure audio, maybe with some credit stills, subtitles or whatever to make it YT-worthy?
Oops, just saw it's for promotional clips. Well, there's always the book cover (if it's that kind of promotion) and stuff like that. Gotta revise my previous point, though. You wnat that imagery to be
connected with your product. And for that, it's better to use exclusive stuff, imho. (Unless it's a historical novel or something. Totally okay to use familiar images then.)