Sir John George Woodroffe (Arthur Avalon) provides a detailed symbolism for the Garland as well as the Girdle in his commentary to Verse Six & Seven of the Sri Karpuradi Stotram.
The excerpts are taken from the book - 'Hymns To The Goddess And Hymn to Kali' by Arthur Avalon.
O Devi! of full breasts, whose throat is adorned with a garland of
heads, They who meditating recite any one or two or three of Thy
very secret and excelling Bijas or all thereof together with Thy
name, in the moonlike face of all such the Devi of Speech ever
wanders, and in their lotus-like eyes, Kamala ever plays.
Whose neck (Munda-sragaiisaya-lasal-kanti)
She who is
S'abdabrahman consisting of 50 letters. Niruttara- Tantra says, A She
is adorned with a garland of heads representing the 50 letters.’
Kamadhenu-Tantra says, "In My throat is the wonderful Blja of 50
letters." Again I worship the Mother the source of the universe
S'abdabrahman itself* blissful. ’.
Visvasara says - 'Blissful Brahman
is adorned with S'abdabrahman and within the body is represented by
O Mother, even a dullard becomes a poet who meditates upon
Thee raimented with space, three-eyed, Creatrix of the three
worlds, whose waist is beautiful with a girdle made of numbers of
dead men’s arms, and who on the breast of a corpse, as Thy couch in
the cremation-ground, enjoyest Mahakala.
Whose loins (Bahuprakarakrtakaticiparilasannitambam)
At the end of each Kalpa all Jivas abandon their gross bodies, and existing in their subtle bodies in which their respective Karmas
inherent form part of the Avidya which is in the causal body of the
Brahmarupinl associated with Her own Gunas (Svaguna) until they are
liberated at some future time after the commencement of the next
Kalpa. Hence the girdle adorning the loins lower belly and generative
organ of the MahadevI vlratrupini, capable of producing children, is
fashioned of the arms and hands of dead Jivas, For these arms and
hands were their principal instruments for the doing or work
The S'aktanandatarariginI says, With Karma is a Jiva born
with Karma he dies and in the next body again that Karma is attached
to him Devigita says, In Her at dissolution Jivas and their Karmas
arc merged in undifferentiated mass, just as all which is done
(Vyavahara) merges in dreamless sleep (Susupti). Again the Devi says,
It is I who create the whole world and enter therein with Prana, Maya,
Karma and so forth.
So the Garland is a representation of the Devanagari letters, She being the source of the sound, and all the Mantras.
The Girdle represents the cyclic Karmic nature of a Jiva who's reborn again and again after each successive dissolution and creation, merging and emerging from and into Mahakali in each cycle.
On a side note, there's a description of Mahakali, provided by Shiva in Chapter 13 of the Mahanirvana Tantra, which although not relevant to this question, nevertheless, does provide the symbolism behind the Saguna Aspect of the Goddess.