Well, it doesn't matter if he is in a slump or not because nothing changes either way. There is only one way a batter gets out of a slump, keep swinging.
And from the play calling, the coaches are not treating him as if he's in a slump.
If he is in one, the good news it is now with a 3 game lead and 4 to play so he's got time to work his way out of this and regain his confidence, if it has been dampened.
Dak Prescott has been doing this long enough to know the magic words "Hey, I've done this before, I can do this again".