Why does every criticism I read of John Steinbeck's Of Mice and Men include a character named Michelle and how she plays a huge part?! I cannot...

That's very odd! Would you be able to post an example quote from these criticisms to show what you mean? (Could it be possible that the materials you're viewing are actually referencing a different piece of literature, perhaps with a similar title?)

The story references Lennie's Aunt Clara a few times, a now-deceased lady who used to take care of Lennie before George did.

But the only female character who plays an important role in...

That's very odd! Would you be able to post an example quote from these criticisms to show what you mean? (Could it be possible that the materials you're viewing are actually referencing a different piece of literature, perhaps with a similar title?)

The story references Lennie's Aunt Clara a few times, a now-deceased lady who used to take care of Lennie before George did.

But the only female character who plays an important role in Steinbeck's Of Mice and Men is Curley's wife, whose own name we never learn. She's desperate for attention, probably because her life is so isolated and lonely, and she constantly flirts with other men.

Here's when we first hear about Curley's wife, when the men are discussing her habit of checking out other guys:

George cut the cards again and put out a solitaire lay, slowly and deliberately. "Purty?" he asked casually.

"Yeah. Purty... but-" George studied his cards.

"But what?"

"Well- she got the eye."

"Yeah? Married two weeks and got the eye? Maybe that's why Curley's pants is full of ants."

 And here's an example of her behavior:

"Any you boys seen Curley?" They swung their heads toward the door. Looking in was Curley's wife. Her face was heavily made up. Her lips were slightly parted. She breathed strongly, as though she had been running.

The other men sense that Curley's wife is trouble. Toward the end of the story, Lennie accidentally kills her, which causes a mob of men to come looking for him for revenge--and that's how Lennie himself dies, put to death out of mercy by his friend George, who wants to ease Lennie into his inevitable death rather than suffer at the hands of the mob.

So, as you can see, Curley's wife is very important to the plot, but there are no other female characters in the story. The only other solution I could suggest for the issue in this question is that perhaps some people prefer to invent a name for Curley's wife while discussing her, uncomfortable with the continual references to her as belonging to someone else.