You have a very generous Mom!
dcandmc is right: Without knowing into what mix of underlying investments the account is invested, it is impossible to say whether the returns were large, small, or in-between. Especially since your grandmother invested a lump sum (as opposed to dollar-cost averaging), the exact day of the investment could have a significant impact on the overall return.
For example, if she had invested entirely in the S&P 500 on Feb 3, 2014, today she would have had a return of 17.58% assuming no fees or transaction costs. If she waited until Feb 18 though, just two weeks later, her return would have been less than 11%. She obviously wasn't invested in the S&P 500, and more than likely was invested in a more conservative portfolio. The low interest rate environment has not been kind to conservative fixed income investors, so a return of less than 2% may be competitive with market rates depending on her portfolio construction.
So my suggestion would be to carefully review the asset allocation in the 529 plan. It sounds like this is an advisor-sold plan, so consider asking your mother to meet with or call the advisor together to make sure that you are both comfortable with its historical performance and current investment mix.