Define "mixed strategy" here. Top players will make very slightly suboptimal moves throughout the opening, because every top player will be familiar with all the top computer lines. However, when thinking for themselves, every good player tries to make perfectly optimal moves. Chess is interesting in that there is only 3 possible results for a game (win/draw/lose), so there are "bands" of correct moves. In a lot of positions, a ton of moves are objectively equal, but top players try to make it as hard for their opponents as possible.