I know that when answering a question, the topic + は is implied and left out of the response because it's redundant. Is the same true for the direct object of the verb + を?
Yes, it is the same.
In a Japanese language class, however, the teacher might expect you to give the full answer 「はい、映画{えいが}を観{み}ました。」.
In a real-life situation, native speakers often just reply 「観た。」、「うん、観た。」、「観たよ。」 , etc. to tell you the truth, but that does not mean that the beginning students should speak like that as well.
The verb is 「観る」 and not 「見る」 for watching something appreciatively. But then again, if you are being taught that it is 「見る」 in a beginning class, just use it for now.