Why did God create mental illness/depression?

He did not. It is the result of Adam sinning and imperfection.

He didn't.  Those are just things that can go wrong in mortal life.

Why do you assume He created it? 

God did not create mental illnesses and depression and diseases God created a perfect world and he placed the first man and woman Adam and Eve in a perfect setting and he told them to have children and to be the Masters over all the animals and to take care of planet Earth and not to eat from the one tree in the middle of the garden.

This tree was doubtlessly and immunizing agent for animals and birds.  But for human beings it was a rogue protein a prion.

God said don't eat from that because you'll become a dying race yes read the original language.  

Man disobeyed God he ingested a food that he was told not to eat and he started all kinds of diseases by doing so.

To this very day man's not paying attention to what God says not to eat well the coronavirus is in our faces because somebody chose to eat something that God said don't eat it.

Oh well it appears man likes to make himself sick so by all means if you like to be sick just go and disobey God you'll get the natural results of doing what is bad

hahaha religous clowns got no common sense,god didnt create depresson, u took alchol and drugs that caused damage to the brain,  a damaged brain cannot work properly and cannot be happy all the time

Deana's answer makes no sense, because surely it is within God's power to fix those things.

If it is within his power (he is all-powerful), then I guess he doesn't mind all the suffering - or even enjoys watching it.

He didn't create it. When Adam and Eve sinned they open a terrible portal to sickness, torture, disability, hunger, abuse, and ultimately death.