By the standards of Top 5 in the world yeah, but not top 50 or 100 or as a professional tennis player overall. He has 17 titles at the moment, and does well at grand slams for sure, if he was mentally weak he would never be at this stage right now.
I reckon if he was playing another player besides the absolute top players right now, he would be able to recover from this dip in form. But this is Nadal and a grand slam semi-final I guess, and even a slight dip is beyond punishing.