No, I wouldn't say he's a traitor. The Empire (or maybe just Vader) kept changing the nature of the bargain and it was obvious that if Calrissian didn't comply, Cloud City could have easily ended up like Alderaan (too soon?) Besides, Lando only really knew who Han and Chewie were. He could have been completely ignorant about Leia and the droids and, there's no way he could have known who Luke was. Lando was looking at it like this; 2 friends and a couple unknowns vs possibly millions of inhabitants in Cloud City.
To top it off, he didn't just redeem himself when he took out the second Death Star, he was backstabbing the Empire before Han was loaded onto the Slave I. He may have known the Empire would have screwed him over but either way he still had his android man-servant and several militant forces ready. After that he threw up the alarm and saved Leia and the Droids.
After that, it must have taken him those three years between Empire and Jedi for him to have gained enough trust and a good enough reputation to have become one of Jabba's in-house guards.
Lando may have sold out Han but he had redeemed himself before he ever left Cloud City.