I know that the Statue of Liberty is made of copper, at least externally, and therefor wasn't always the famous green color that we associate with it now, as this was a result of years of oxidation. Was this planned? Or is this the result of laziness/being too impractical to weather proof?

The oxidized patina on the statue of liberty is extremely durable and weather resistant, so it itself is a form of weather proofing.

This would have been known at the time.

The statue is also just coated with a very thin layer of copper, so it's highly unlikely this was an accident. (My personal opinion)