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.

That is how I would interpret it

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.