It is a looser translation of it. He lā nani. or He lā nani ia. would be It's a nice day. However, that does not take into account the word kēia. So yours is not as precise.
In past lessons I've noticed adjectives mostly after the noun. Why is it before the noun here?
To modify a noun, it would go after. However, this is a complete thought - What is today like? Today is beautiful. It is a complete qualitative statement. If you put nani after, it would translate to "this beautiful day" which technically is not a complete thought.