"I don't agree with the comment above."
The difference here is that it's not used as a pre(/post)position, but as an attribute - it is indicated by 的 which generally stands for "of". So our Chinese phrase 上面的评论 literally translates as "the comment of below".
If however we use 上面 as preposition here (with or without the optional 的), then yes, it should have been behind: 评论(的)上面, but then the meaning would become "under the comment" which does have its own meaning but doesn't correspond to the intended meaning of the whole sentence, and also makes the whole sentence wrong in two aspects: 1. Prepositional phrase should go before the verb, and hence 2. The sentence becomes incomplete as the transitive verb 同意 now lacks the direct object.