In Chapter Seven of Chamber of Secrets we have the following statement:
“It’s about the most insulting thing he could think of,” gasped Ron, coming back up. “Mudblood’s a really foul name for someone who is Muggle-born — you know, non-magic parents.
Given that Ron defines a Mudblood as someone with non-magic parents, the implication would be that non-magic grandparents do not make someone a Mudblood.
On the other hand, the underlying idea of blood-status is that some people diluted their pure wizard blood with Muggle blood. If your blood is merely diluted then you’re a Half-Blood, but if you’re blood is entirely Muggle then you’re a Mudblood. If all four of your grandparents are Muggles then your parents likely had no wizard blood. If that’s the case then (particularly in the last book where Muggle-Borns are thought to have stolen magic from wizards) the fact that your parents are wizards wouldn’t purify your blood. It would stand to reason, then, that you’d still be a Mudblood.
Of course, neither of these arguments are definitive.