Your question is asking about my favorite line of text in the entire book.  

Had I ever harbored the mystical notions about mountains that seem to obsess lawyers and judges, Aunt Alexandra would have been analogous to Mount Everest: throughout my early life, she was cold and there.

The line cracks me up every single time that I read it.  It is just so blunt.  On top of that, it is one of the most completely unflattering lines of text written about a woman that I have ever read.  What woman wants to be compared to a mountain?  Scout does that with her Aunt Alexandra, but she doesn't choose just any old mountain.  She chooses the largest and most daunting mountain the world.  

I just finished reading John Krakauer's book Into Thin Air, and it retells his experience climbing Mt. Everest.  As terrifying as the mountain is, there is no doubt that the mountain itself is beautiful and awe inspiring.  But again, Scout doesn't relate Aunt Alexandra to the nice parts of Everest.  Scout simply says that she is "cold and there."  Like a giant lump of cold heartlessness.  

The reason Scout compares her Aunt to a giant, immobile mountain is because Aunt Alexandra is a strict household ruler.  Atticus has her come to Maycomb to help out with the house while he is working the Robinson case.  She comes, but she immediately brings her domineering and mountainous spirit to bear.  She tries to control what Scout does, says, and wears.  Aunt Alexandra is a giant mountain blocking Scout off from everything that she is used to doing.

Aunt Alexandra was fanatical on the subject of my attire. I could not possibly hope to be a lady if I wore breeches; when I said I could do nothing in a dress, she said I wasn't supposed to be doing things that required pants.

In what ways is Aunt Alexandra like Mount Everest in To Kill a Mockingbird?

Scout's Aunt Alexandra is somewhat larger than life, and she obviously rules the roost at the family home at Finch's Landing. She wears the pants in the family, and "never saw any reason to take notice" of her husband, Jimmy. Alexandra's son, Henry, "left home as soon as was humanly possible," and rarely visits his parents or the old family homestead. Her grandson, Scout's cousin Francis, is spoiled and obnoxious, and Alexandra's pampering has turned him into a sissy who hides behind her whenever he antagonizes the tomboy Scout. Alexandra's mothering skills are highly questionable, and though she believes Jem and Scout needs a woman's touch, it is clear that Atticus is far superior when it comes to parenting abilities. Alexandra considers the Finch family part of Maycomb's royalty, and that their "gentle breeding" sets them apart from other Maycomb families. She still uses a Negro chauffeur and attempts to take over as head of Atticus's household when she arrives in Maycomb. But she fails to convince Atticus to fire Calpurnia, and Atticus finally stands up to his sister, firmly telling her that Cal is a beloved member of their family. Alexandra reminded Scout of Mount Everest: always "cold and there."

She was not fat but solid, and she chose protective garments that drew up her bosom to dizzying heights... From any angle, (she) was formidable.  (Chapter 13)