Learn French: Une nuée de commentaires sont apparus sous celle-là. - A swarm of comments appeared under that one.

When a collective noun like une nuée de is followed by a plural noun (commentaires), French often lets you choose the agreement. You use the singular to focus on the group as a single mass, but you use the plural (as done here) to emphasize the large number of individual things acting.

Since the verb agrees with commentaires, why is the participle apparus and not apparues?

The noun un commentaire is masculine, so the plural past participle is apparus. If the speaker had decided to emphasize the collective noun and make the verb agree with the feminine une nuée, it would have been written as est apparue.

Does une nuée literally mean a swarm?

Yes, it is related to un nuage (a cloud) and traditionally refers to a thick, moving cloud or a massive swarm of birds or insects. Used figuratively here, it gives the vivid image of an overwhelming, buzzing flood of comments.

Why use celle-là at the end instead of just ça?