Can You Be Considered Negligent for Not Wearing a Seat Belt?

No, and here is the Massachusetts law on the subject:

Exclusion of Seat Belt Belt Evidence
 
"Failure to wear a properly fastened safety belt shall not be considered as contributory negligence or used as evidence in any civil action." G.L. c. 90, § 13A, as added by 1993 Mass. Acts 387, § 3.