It does seem that Mugabe may be taking advantage of Vatican invitations to lend legitimacy to his reign. He also showed up at the beatification of John Paul II in 2011. Perhaps the Vatican should be more discriminating in it's invitation list - but perhaps not. If it makes a point of inviting every nation, then it avoids making a political statement by its list. Did it invite Palestine? That, to me, would be a more telling issue.
Still, since Jesus ate with sinners, maybe it's not appropriate for a church to vet the invitation list.

-Joe-