Translation:The frog is ugly.
Think of it as "The frog turned out ugly." There are many similar instances in the language. After eating, "배가 불렀습니다. "OK" is usually expressed eiher in the past or the future, 알았다 or 알겠다, but rarely in the present. "No thanks," "don't bother," "Don't even go there" can all be expressed by 됐다.
This verb has to be put in the past tense.
I copy paste from here.
Yes. It's because they are derived from the verb 잘/못 + 생기다. 생기다 = for something to happen that didn't happen, for something to appear in a certain way.
잘/못생기다, it takes the 2nd meaning. For a guy to look good/bad, he already has to appear in a certain way, so the past tense is used. It already has to have happened.