Should ex-Presidents dabble in other countries' elections?

I don't see a problem as long as they are being open and honest about it. It's only a problem if they are doing stuff under the table.

I think the better question should be should presidential candidates accept meddling by other countries in our presidential elections?

While probably not the best of ideas, he is a private citizen with the right to do what he wants. Plus it is better than meddling in foreign elections while in office like quite a few former Presidents have done.