He was pretty arrogant before the RoSaT as well, when he achieved SSJ. He gave two fuckings towards Trunks' warning and decided to go out there and face the 3 androids alone, and later stated they should allow Cell to become perfect so it'd spare time considering they'd have just one foe to kill, even though (as pointed out by Piccolo) he couldn't even handle one android alone.
I agree that he seemed way less of a cocky bastard and a more pragmatic one in the Freeza saga and whilst it's possible Akira Toriyama was just not consistent with his personality, you could say that him turning SSJ just made him cockier than he used to be, as according to his Freeza's saga monologues he believed the SSJ was the peak of power that no other warrior could reach, and even Piccolo -- after Vegeta losing to #18 -- said he recovered his confidence after becoming a SSJ.