Is it a sin to like diapers?

Diapers are nothing more than an object created to fulfill a function. They are similar to toilet paper or other hygiene "tools." I am sure most people would like a baby to be wearing a diaper when they were holding them. If they baby urinates, it is more hygienic than having a urine soaked shirt. There is nothing sinful about it.

If your question is referring to something more, such as fetishes for instance, then the answer may change depending on the "like" and your beliefs.