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It's odd that the mail carrier hasn't come yet.
Literal
Postal-delivery-person [subject-が] not-yet come-[negative-ていない]-[nominal-のは] odd.
まだ + negative is 'not yet.' The structure ~のはおかしい nominalizes the preceding clause and predicates a judgment ('the fact that X is strange'). おかしい covers a wider range than English 'strange': it can mean 'odd, funny, suspicious, wrong.' A compact complaint about a routine that's been broken.