Why would you rat out someone for not wearing a seat belt? Are you that bored, or just dislike it when someone decides to disobey some misplaced authority?

I've never needed a belt extension, but it must be very embarrassing to have to ask for one. I've been in seats where I have a lot of belt leftover, and I've been on some where the belts are much shorter. It varies from plane type to plane type and even seat to seat, especially on regional jets.

Generally speaking, if it doesn't negatively impact you, you shouldn't say anything. You may do something that technically violates some airline policy like bringing more than one personal bag in your carry-on, or something of the sort. Or maybe you have body odor, or bad breath, or a crying baby, or something that actually does have a negative impact on other passengers.