Is "to a good approximation" idiomatic?

That is correct, idiomatic English, but depending on the exact meaning, you may be looking for "to a first approximation", meaning:

a roughly approximate value of a quantity often preliminary to more precise determination

The difference is nuanced, but my impression is that "to a good approximation" refers to a somewhat more exact approximation than "to a first approximation" (in that one could make better approximations after the first).

Google Ngrams Viewer suggests that both are used in writing, with "to a first approximation" being somewhat more common. Also, both are declining in usage in modern writing.