Let me start with listing a few themes in the poem. Sex and sexuality, femininity, narcissism, and vanity. Those are not the only themes present in Pope's poem, but when a reader thinks about Belinda, those themes jump off of the page fairly quickly. She loves her looks and cares deeply about her beauty. Honestly, if Belinda was doing her toilet scene in modern day times she would be taking selfies and posting them to Instagram the entire time.
Pope probably gives one of the most high brow descriptions of a woman getting ready to go out in history. At its core, the toilet scene is nothing more than describing a woman putting on make up, doing her hair, and getting dressed. One way that the section is significant is that it shows Belinda's wealth. She has servants helping her get ready. One is actually mentioned by name -- Betty. When the poem was written (and even today) having servants to help get you dressed was not the norm, unless you had a high degree of wealth.
The busy Sylphs surround their darling Care;
These set the Head, and those divide the Hair,
Some fold the Sleeve, while others plait the Gown;
And Betty's prais'd for Labours not her own.
The main significance of the toilet sequence though is that it highlights just how vain Belinda is. She is doing everything she can to further highlight each and every aspect of her already initial beauty. Her beautification project is even likened to a form of worship. Notice the words "priestess" and "altar."
A heav'nly Image in the Glass appears,
To that she bends, to that her Eyes she rears;
Th' inferior Priestess, at her Altar's side,
Trembling, begins the sacred Rites of Pride.
That's about as close to narcissism as I can recall in any poem I've read. The toilet scene closes with Belinda putting on the rest of her armor.
Now awful Beauty puts on all its Arms;
Her getting ready process is compared to getting ready for battle. Crazy. People put on armor to win battles and protect themselves. Even save their life. To Belinda, that's how important it is that she looks good.