Explanation

People believe that donkeys are the stupid creatures means لوگوں کا ماننا ہیکہ گدھے احمق مخلوق ہیں

 It is believed that donkeys are the stupid creatures مانا جاتا ہیکہ گدھے احمق مخلوق ہیں

Sometimes we have to skip "People" while converting the sentence into passive.

Present Indefinite Tense

Active Voice: Subject + Verb + Object

Passive Voice: Object + is/am/are+ third form of a verb + by + subject.