is it safe to hide bad comment?
Could you please be a little bit more descriptive?
I imagine Facebook is wise enough to know that ads having a large % of comments hidden/removed are causing negative user experience.
It will be fine to hide stuff, just realise that hiding everything on ads with a lot of volume is sure to NOT play in your favour.
Always put yourself in Facebook's shoes. If you saw ads with lots of comment hiding, what would you think if your job was to find low quality ads?