R.A. Salvatore's novelization did NOT mention any surprise:
And then it happened, all of a sudden. Mace started to slash to the left, cut it short and stabbed straight out, then reversed his grip and sent the lightsaber slashing across, left to right. He spun a complete circuit, coming around to parry a blaster shot, but there was no shot forthcoming.
That left to right reversal had cleanly landed. Jango Fett's head flew free of his shoulders and fell out of his helmet, to settle in the dirt.
Neither does the screenplay:
Finally, FETT is free and kills the REEK. MACE WINDU fights fiercely with
JANGO FETT. Finally, the bounty hunter falls. His helmet
goes flying. The bounty hunter's body falls to the ground.
As far as the movie, it's not surprise on Master Windu's face, I think, but sadness (I'll try to see a formal explanation for it in Shatterpoint novel later):
Also, Dooku is merely looking unhappy, not surprised:
Master Windu's mood is explained in Matthew Stover's C-canon novel Shatterpoint, which centers on that character's story. He felt in the Force that what he must have done was to kill Dooku. Instead, he did not, and that permitted the Clone Wars to start, with no second chance to stop it. He's not sad at Jango's death, but at what that death means as far as signaling his failure.
In my dreams... My blade doesn't light the underside of Fett's square jaw. I don't waste time with words. I don't hesitate.
In my dreams, the purple flare of my blade sizzles the gray hairs of Dooku's beard, and in the critical semisecond it takes Jango Fett to aim and fire, I twitch that blade and take Dooku with me into death.
And save the galaxy from civil war.
I could have done it.
Leaving that aside, there's is reason for many (any non-Jedi) to be surprised:
Windu himself has a healthy appreciation of Jango's deadliness. From Shatterpoint again, here's Master Windu's thoughts about Geonosian arena (from after the fact remembrance):
Jango Fett bristles with weapons. An instinctive killer: the deadliest man in the galaxy. Jango can kill me in less than a second. I know it. Even if I had never seen Kenobi's report from Kamino, I can feel the violence Jango radiates: in the Force, a pulsar of death.
We see that less that 5 minutes prior, Jango Fett easily killed another Jedi (A Jedi Master, and a member of Jedi Council to boot - Coleman Trebor) with his blaster, one on one. Here's the youtube clip of the event, and a small image montage I made from my copy of AotC: