The official novelization suggests that Obi-Wan was more taken aback than the film lets on; it also suggests that he was deliberately concealing his feelings, which would presumably include any consternation about the Kaminoans' appearance (emphasis mine):
Obi-Wan pushed back his hood, which had offered little protection from the driving rain, and brushed the water from his hair. Wiping his face, he turned to face the speaker, and then he paused, caught by the image of the Kaminoan.
The words finally distracted Obi-Wan from his bemused perusal of her strangely beautiful physique. "I'm expected?" he asked, doing little to hide his incredulity. How in the galaxy could these beings possibly have been expecting him?
Obi-Wan nodded and tried to play it cool, hiding the million questions buzzing about in his thoughts. After all these years? They were thinking that I wasn't coming?
Attack of the Clones: Official Novelization Chapter 15
Bear in mind that the Kaminoans don't know that Obi-Wan is unfamiliar with them; as far as they know, he's a representative of one of their clients and, even though he may have never visited their planet before, they have every reason to believe that he's at least theoretically familiar with them.
For his part, Obi-Wan doesn't want the Kaminoans to think he's uncomfortable. He's very explicitly playing along with their mistake to pump them for information, and any suspicions he raises in their minds is going to jeopardize his mission. It would rather give the game away if he's openly gawking at them.
Though, all of that being the case, I would deeply question the assumption that he should be unduly shocked by their appearance. As I (and others) have mentioned in various comments: sure, he's meeting an entirely new species, but it's not like there aren't stranger-looking creatures roaming the galaxy. As a well-travelled galactic citizen, we can presume that Obi-Wan has seen his fair share of unusual alien species, and the Kaminoans are a relatively tame design; as noted in a different question on the site, they're not wholly dissimilar from other species he would be familiar with.
What's more, there's some indication that openly gawking at aliens is considered a social faux pas; consider this exchange from the canon novel Thrawn, where three characters discuss the titular alien (who is of a race never before seen in the galaxy at large):
"I've never seen anything like that before," Driller continued. "What is he, some kind of Pantoran with an eye condition?"
"Now, that's just rude," Juahir chided him. But she was staring at the strange being just as hard as he was. "Arihnda? Any ideas?"
"Sure," Arihnda said. "Let's go over and ask."
Juahir's gasp was audible even over the hum of conversation filling the ballroom. "You're kidding."
Thrawn Chapter 10
Although this is admittedly set in the Imperial era, many years after Obi-Wan's encounter with the Kaminoans, this doesn't seem an unreasonable social rule for any period of galactic history, especially in the relatively cosmopolitan years of the Republic.
And if there's anything we know about Obi-Wan, it's that he's unfailingly polite.