Frame rate can affect the displayed ping value. The client's highest priority is to render the game. After it's done rendering the game, it checks for ping packet and calculates ping value. So if the ping packet has already arrived, but the client is still rendering the game, when the client goes to retrieve the ping packet, it will not know that the ping packet has been sitting there for a while. All it can do is (incorrectly) assume that the ping packet had just arrived, and calculate the ping value based on that.
So if the frame rate isn't nice and smooth, it's fair to assume that the displayed ping value may not be accurate.
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