'hardly' qualifies a statement by admitting that something is true, BUT that this is to such an insignificant extent as to be negligible.
This is taken further when we express surprise or disagreement with a statement, and is interpreted as meaning 'no/not' as in, "I hardly think so" = "I don't think so."
So, in your sentence, "It's hardly surprising, is it?", the first clause is virtually the same as saying, ""It's not surprising, is it?" - hence, the tag in the affirmative.
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