Them both belonging to the Akatsuki does not mean anything. In fact, even in the Akatsuki, Deidara is held high in the ranks since Pein considers him to be special as well as tells everyone to mourn his death and Kisame calls Deidara quite strong, despite laughing at Kakuzu and Hidan after they died. That's like saying Itachi or Kisame aren't out of Hidan's league either just because Hidan is an Akatsuki just like them.
Never said Hidan is weak. He just gets rocked by Deidara. Pretty hard at that. Not only does Deidara have all together better reaction, intelligence, compatibility to the enviroment, as well as enemy's fighting method, and manipulative feats than Hidan could ever have, Deidara's AoE and combat capability easily makes Hidan seem like a dumbass in comparison. Nevermind that Deidara's ability caught Akatsuki's interest and they sent out Itachi, Sasori and Kisame to recruit him when he was only 13-14 years old. Only other Akatsuki member that joined at such a young age was Itachi. Hidan was recruited because Kakuzu kept slaughtering his partners and Hidan can't be killed by him.
Deidara understimated Sai and was all around arrogant due to fighting someone who is younger than himself and also uses art as a medium for his fighting style - considering Deidara considers his art style to have no flaws. He jobbed that one fight. So what? All of his other fights have much better feats and combat capability than that one fight.
No, he doesn't. Zetsu canonically didn't know anything about Sasori past Hiruko. He was interested in how Sasori's real body looks.
