Watt jumped the snap count, don't feel there was much Smith could do on the play that Romo pivoted and found Williams for the TD.
Murray's execution on a chip with Bruce Irvin was horrendous, in fact, Murray chipped more of Tyron Smith than Irvin on the play.
I'm assuming these are examples OP is talking about with regard to Smith's play.
Now yesterday, JPP got the better of Smith no doubt. When motivated JPP is one of the most devastating DEs in the game. A lot of tackles, including those as talented as #77 are going to have days like that against him.
Playing LT in this league is by no means easy, Smith is developing into one of the best. He won't be 24 until December. On the list of Cowboy concerns, he's way down the list for me.