I don't think it is, based upon the following observations:
(assuming the object is not a defect/scratch/etc. in the film)
1) The streak is travelling in a straight line - not an arc - therefore it is moving at very high speed (not to mention the fact that it's not in the first frame - and a streak, far from the President, just one frame later).
2) Drawing a line through this straight streak, and continuing to draw it over JFK's head, the object ends up about 2 feet over JFK's head.
3) Even if we assume that the object is travelling at an angle to or from the camera, a line drawn to/from the object and JFK does not intersect with JFK.
Therefore, unless the object went vertical for some distance BEFORE continuing in a straight line away from JFK's head - I don't see how it is a piece of JFK's skull.
Just my .02 cents*
*(and not trying to guess at what it could be.....because I don't have a clue. Conspiracy theorists might say it's another bullet - due to reports of multiple gunshots fired (where's the grassy knoll in this film? To the right, correct? Hmmmmm....... OKAY,I am NOT going there! lol)