Learn Italian: Il clima qui è tanto fresco quanto in montagna. - The climate here is as cool as in the mountains.

Yes, absolutely! When comparing adjectives like fresco, tanto... quanto and così... come mean exactly the same thing ("as... as"). You can use either pair interchangeably in this sentence.

Do tanto and quanto need to change endings to match il clima?

No, they don't change here. Because they are paired with an adjective (fresco), they act as adverbs and stay invariable. You only change their endings to match a noun when comparing quantities of things (like "as much water as wine").

Why does the Italian sentence use the singular in montagna when the English translation says "in the mountains"?

In Italian, it is standard to use the singular word montagna to refer to "the mountains" as a general environment or destination. The set phrase in montagna is the most natural way to express being "in the mountains" or going "to the mountains".

I thought fresco meant "fresh". Why is it translated as "cool"?

When talking about weather, air, or temperature, fresco means "cool" (a pleasant, slightly cold feeling). It does mean "fresh" when applied to food, like pane fresco (fresh bread), but here it describes the climate.