El tiempo no podía haber sido mejor.

English Translation

The weather couldn’t have been better.

Is tiempo the accurate word for weather in this instance?

I would say so. You will also see clima for weather, but that can be misunderstood for “climate”.

Regarding this sentence, it strikes me as a bit odd, since you wouldn’t normally hear “El tiempo es bueno” (about the weather), but rather “Hace buen tiempo”.

Disclaimer: I’m not a native Spanish speaker.