After Barbara linked to the Pope poem I decided started to read it. I haven't finished it, but what is clear is that Belinda definitely over-reacts to the theft of her hair.
I was reading it from an abridged version in Adventures in English Literature from 1931. In the introduction for it, I was surprised to learn that this poem arose from a real incident.
From the introduction
It happened that a certain foppish young baron named Lord Petre had cut off a curl from the hair of Miss Arabella Fermor and refused to give it up. Out of this trivial incident there arose between the two families a quarrel which threatened to assume the proportions of a feud. A friend of Pope's named Caryll suggested that the author write a poem to show the absurdity of all this to-do. Pope therefore wrote a "mock-heroic" poem.
To quote Barbara from that thread (now archived- Admiring Pope no more than was proper) "Because he does this without her permission, she becomes very upset, and a battle ensues".
I haven't even got to the ensuing battle, her despair over the lock goes on and on. Here are some funny bits
The meeting Points that sacred Hair dissever
From the fair Head, for ever and for ever!
Then flash'd the living Lightnings from her Eyes,
And Screams of Horror rend th' affrighted Skies.
Not louder Shrieks to pitying Heav'n are cast,
When Husbands or when Lap-dogs breath their last,
She sighs for ever on her pensive Bed,
Pain at her side, and Megrim at her Head.
Her Eyes half languishing, half drown'd in Tears;
On her heav'd Bosom hung her drooping Head,
Which, with a Sigh, she rais'd; and thus she said.
For ever curs'd be this detested Day,
Which snatch'd my best, my fav'rite Curl away!
Happy! ah ten times happy, had I been,
If Hampton-Court these Eyes had never seen!
A Sylph too warn'd me of the Threats of Fate,
In mystic Visions, now believ'd too late!
See the poor Remnants of these slighted Hairs!
My hands shall rend what ev'n thy Rapine spares:
These, in two sable Ringlets taught to break,
Once gave new Beauties to the snowie Neck.
The Sister-Lock now sits uncouth, alone,
And in its Fellow's Fate foresees its own;
Perhaps Elinor, knowing this poem, believing Edward took a lock of her hair, decided that it wasn't that big of a deal. I would think Edward stealing a lock of Elinor's hair is creepy, but compared with how Belinda over-reacts to the theft of her hair, well...
In light of this poem, it seems Jane Austen was using the locks of hair in S&S for specific purpose.