He can't, of course, seize TikTok. But what he can do is block its use in the US - presumably by Executive Order on grounds of national security. I'm no US constitutional lawyer, but the Donald does seem to have done a lot over the last 3.5 years by Executive Order. More seriously, he's proposing to do the same to WeChat, which is how many Chinese, outside China as well as in it, communicate with each other.
Incidentally, in terms of TikTok showing more than Asian teens dancing badly, one thing it's featured quite a lot of recently is videos on systemic/structural racism. There's a nice collection on Youtube:
https://uk.video.search.yahoo.com/yhs/s ... ction=view. And I personally know that that kind of thing REALLY pisses off Trump supporters.