Whether the theft of registration tags is a felony or misdemeanor is a somewhat tricky question. As you might expect, this will vary by state. You can ultimately expect however that it will be regarded as a form of larceny.
Identification
First, stealing from a motor vehicle is entirely different from stealing one. Most states will then differentiate between auto theft--which is usually a serious felony--and stealing the contents or appendages to an automobile. This means that stealing registration tags will generally mean a lesser larceny charge.
Larceny
Larceny charges usually fall into several categories ranging from petty larceny to grand larceny. A state may decide upon the category based on the value of the object stolen. The amount will vary depending on the state. In Virginia, for example, any crime where an object of less than $200 is stolen is deemed petty larceny.
Misdemeanor or Felony?
Since registration tags usually cost much less than $200, it will be petty larceny in those states that handle the issue by market value. This is typically a misdemeanor. In New York state, it is a class A misdemeanor. However, some states may decide that registration tags represent official state records or property and thus charge the defendant with a more serious offense.