Learn Croatian: Ostavili su loše komentare.

It is in the accusative masculine plural case to match the noun komentare. Because loš ends in a soft consonant (š), it takes soft endings. For masculine accusative plural, the soft ending is -e, which neatly matches the noun ending.

Why use the perfective verb ostavili instead of the imperfective ostavljali?

The perfective ostavili shows that the action of leaving the comments is a single, completed event. If you used ostavljali, it would mean they were in the process of leaving comments, or that they were leaving them repeatedly over a stretch of time.

Since masculine plural nouns usually end in -i, why is it komentare?

Masculine plural nouns do end in -i, but only in the nominative case (the subject of the sentence). Because the comments are the direct object of the verb, they must be in the accusative case. The accusative plural ending for almost all regular masculine nouns is -e.

Could we put the clitic su at the end of the sentence?

No, su is an enclitic and strictly follows the second-position rule in Croatian. It must come right after the first stressed word or phrase, which in this pro-drop sentence is Ostavili. If you explicitly included the pronoun Oni (They), the sentence would become Oni su ostavili loše komentare.