This is the standard charge that liberal activists make against Justice Thomas. He might be the best legal mind currently on the court.
There is ample evidence that Sotomayor is not up to the job of being an appelate court judge, much less a Supreme Court justice.
I am biased toward thinking Justice Scalia is the best legal mind on the court, but legal commentators (on the left and right) have described Justice Thomas as the best legal writer (i.e., clear with tight reasoning).
Given her past record, Ms. Sotomayor will likely be the worst writer on the court in many years, unless she picks unusually talented clerks.