In your particular situation, it seems this couple started "tacky" a long time ago. Although, I would caution you that your MIL is perfectly capable of fighting her own battles (or choosing not to), and it can be quite risky to get involved. You only indicate you're annoyed, not directly involved, but it never hurts to mention!
But, the larger discussion seems to have left out the entire point of giving a wedding gift. When a very young couple gets married (or has a baby, for that matter), they certainly need help setting up a household. But, the main point of sending a gift, particularly in this age when many people are older when they marry and have likely lived outside of Mommy and Daddy's house for some time, is to celebrate the wedding! How many of you give your kids birthday presents because they NEED another new toy? Of course not! You're happy that it's their birthday and want to make it a special day. If, as some have mentioned, you're not "happy" about giving someone a gift (for any occasion), then I would suggest that you simply shouldn't. It defeats the entire purpose and adds resentment to the relationship. I would also go so far as to say that if you are that unhappy about the situation, it is probably not appropriate for you to go to the wedding, either.
As far as the registry goes, there are multitudes of people who think any type of registry is tacky. I, for one, think absolutely any mention of a registry on the correspondence (even shower invitations, but especially the wedding invitation itself) is horribly tacky. The same goes for inviting anyone to a shower who isn't invited to the wedding itself (the only exception being a very tiny ceremony with many guests just being invited to the reception). I am 100% against any situation that may come across as "asking" for gifts. However, there will likely be people who ask either the bride and groom or their family what they need/want for a gift. In that case, I think it's not only appropriate, but helpful to mention a registry of whatever sort. Honeymoon help, household items, whatever the couple would like.