Names on headstones are often misspelled, and many times a nickname or the "known as" name is used. 

The name מתת (Mates) is a partial name of מתתיהו (Matisyahu), which is usually pronounced Matis or Mates. 

Note: In some accents the letter S in the name is pronounced T, hence Mr. Rosen's translation above Matatyahu. 

--
Yitschok Margareten