This weekend we attended a small, lavish wedding. Is it corny or weird to write a thank you note to the parents of the bride for inviting us?

Tj, it is not corny or weird to write a thank you note. It happens to be a very thoughtful way to thank the bride's parents and they will appreciate your kindness tremendously.

Agree with Ruby PItbull.

It is always thoughtful and will be appreciated when you write a thank-you note.
It's just very polite.
Which is a good thing :)

I agree with all of the above. In addition, a lot of people don't take the time to write thank you notes. So, I'm sure that you writing one will be greatly appreciated. It will make for a fine keepsake in someone's album concerning the wedding.