In order to get a better gravestone on the Sims 3, you have to make sure that your sim had a fulfilling life. Examples: Completing lifetime wish and just getting everything they wanted. My sims have HUGE headstones.
*hint*: Its easier to fulfill a dream if you don't have that many sims in your household
*hint*: Its easier to fulfill a dream if you don't have that many sims in your household