That's right. The most common order is (noun + adjective), but that can be reversed in many situations. In this one, however, it's typically not reversed, so it sounds awkward.So, "una mejor excusa" would be correct as well, but it doesn't sound as natural?
No, the Vocabulary forum is for the translation of words, phrases and idioms -- in other words, it's for questions about meaning. Questions about word order, sentence structure, parts of speech and other questions should go in the Grammar forum. I've moved this thread to Grammar.And I posted this in vocabulary because I was just dealing with "Words, phrases and idioms." So I guess it's in the right place?