The problem is they could pull a Facebook and show certain content to change peoples moods intentionally. It's already proven possible. That said, I'm all for legislation against all social media companies.
Changing their mood would be one thing, though I doubt it'd work on TikTok as well as it might on FB, people go to TikTok for specific types of content. Propaganda would be quite another, TikTok's format doesn't seem to really work for that kind of content, it's short videos, not infographics and texts.