Here's a breakdown of Smaug's bewilderment:
* Bilbo's audacity: Smaug is used to being feared and revered. He expects fear from anyone who dares enter his lair. Bilbo's lack of fear and willingness to talk back is completely unexpected.
* Bilbo's cleverness: Smaug is surprised by Bilbo's ability to outwit him, especially with his clever use of the riddle. Smaug never expects to be stumped by a riddle, let alone by a "little hobbit".
* Bilbo's luck: Smaug is frustrated by Bilbo's unexpected good fortune. He's surprised that Bilbo manages to steal a cup from under his very nose, a feat that seems almost impossible.
* Bilbo's survival: Smaug is puzzled by Bilbo's ability to survive his attacks. He expects to incinerate the hobbit easily, but Bilbo somehow manages to escape, despite being small and seemingly insignificant.
Smaug's underestimation of Bilbo is ultimately his downfall. He never expects such a small creature to be so cunning and capable. This underestimation allows Bilbo to exploit his vulnerabilities and ultimately contribute to Smaug's demise.