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Having deep knowledge of Japanese culture is a good thing.
Literal
Japanese-culture [to-に] erudition deep [thing-こと] [topic-は] good thing is.
A moralistic observation. 造詣 (ぞうけい) is a formal noun meaning 'deep knowledge,' 'erudition,' 'scholarship' — specifically the kind of deep learning gained through sustained study of a field. The fixed collocation ~に造詣が深い ('to have deep knowledge of X,' 'to be deeply versed in X') uses 深い ('deep') as the predicate. Here it's used attributively without the が: 造詣深い ('deeply-versed'), which modifies the implied subject. The predicate いいこと ('a good thing') bundles the whole proposition ('being deeply versed in Japanese culture') as a moral positive.