Do kills immediately after victory/defeat count towards your match total?

No, they do not. I have tested this myself, as I got killed at the very end of the match after the Victory animation came up, and I was kind of upset because I hadn't died all match (gotta keep that perfect KDA). Then I checked my stats, and I had no deaths for the match. So no, killing or dying after the victory/defeat animation starts does not count towards the stats for the match.

Note: As @Schism states, this is true for all stats. For instance, getting hooks as Roadhog after the match end does not increase your hook percentage or count.

A minor caveat to Dragonrage's answer. On king of the hill maps, kills after a round ends, but not a match end will add towards your total, from testing it myself. However, as Dragonrage has said, kills after a match end will not count towards your total.