There is a disagreement as to when Yisro's visit actually occurred. One opinion is that this parsha is in chronological order, and thus his visit occurred before matan Torah, while another opinion is that his visit occurred some time after matan Torah.
Q: What would compel anyone to suggest that this parsha is written out of order?
Parenthetically, this is a question we must ask ourselves whenever it is posited that a certain parsha in the Torah is chronologically out of order. While we are taught that אין מוקדם ומאוחר בתורה (the Torah is not always in chronological order), this does not mean that the Torah is a jigsaw puzzle for us to rearrange at will. Unless there is a tradition or an otherwise compelling reason to believe otherwise, we must assume that the Torah is in chronological order, for it would be senseless for it not to be.
So, again, what would lead anyone to suggest that this particular narrative occurred after matan Torah?
A: The immediately preceding parsha contains the battle with Amalek. The nation of Amalek waged war with us without any provocation; they were not threatened by us, and in fact travelled a great distance to the desert just to pick a fight with former slaves. Why did they do this? Because Hashem had revealed to the world that we were His special nation and that we were to be His ambassadors on this earth. Amalek, as the root of all evil, was determined to fight us to the death, and thereby “eliminate” Hashem from this world.
Consider: Hashem had just performed the most awesome miracles, revealing Himself to the world in an obvious, overt fashion. Yet how did Amalek react to this revelation? By waging war against Hashem's ambassadors and, by direct extension, Hashem Himself.
A casual reader of this narrative might easily come to the conclusion that the nations of the world are utterly hopeless. What more could anyone do to get through to them, to demonstrate to them that Hashem is, was, and will always be in charge? Yes, we might easily come to the conclusion that all the goyim are utterly hopeless like Amalek, and thus, like Amalek, must be entirely eliminated.
The Torah informs us that this is not the case by juxtaposing the story Yisro with the story of Amalek. Yisro witnessed the same events that Amalek witnessed, yet he reacted in an entirely different fashion, in an entirely positive fashion. Yisro is clear proof that there is indeed hope for the nations of the world, and it is the continuing mission of the Bnei Yisrael to actualize this hope. It is Amalek that is the exception; for all others there continues to be hope that they will choose the path of Yisro.
While it is unresolved when the story of Yisro occurred, there is indeed a compelling reason to suggest that the Torah recorded it out of chronological order, to drive home this most fundamental lesson.