Olorin the Maiar

[Sharkû]

"Why, do you think, Saruman was accounted the mightiest of the wizards when Olorin was the wisest of the Maiar?"

Gandalf _was_ mightier than Saruman. However, it can be interpreted to signify that Saruman had a higher reputation and standing in Valinor, not based on actual greatness of spirit (which Manwe, Varda, and apparently Círdan perceived).

"WAs it because of Saruman's cunning? Or was it a design of Olorin's?"
Perhaps partially. Gandalf was in fact very humble.

"Also, were the Maiar bounded by wizardly powers and mortality when they were in human form? (Correct me if I'm wrong, but Maia can't die)."

I am unsure what you mean by 'bounded by wizardly powers'. We do know that their powers in their incarnate form as istari, wizards, were limited (either by a moral code which Saruman for example broke, or actual restraints).
In their incarnate forms, the wizards could die like a man; Gandalf did, and was only revived by Eru himself.