S. R. Faruqi:

naḳhl-e mātam = A bier [tābūt]; the decoration that is put on biers.

The mood of the verse is worthy of praise. This verse is a warning flag to those people who call wordplay foolish or artificial and unaffecting. The whole verse is based on the wordplay of the 'tree of mourning' as bringing 'fruit'. The 'tree of mourning' is also called a 'tree of Muharram' or a 'tree of patience/endurance [ʿazā]'.

The adornment that was put on a coffin presumably used to include the shape of a sword, as is clear from this [Persian] verse of Ashraf Mazindarani's. This verse is included in both [the dictionaries] bahār-e ʿajam and mat̤liḥāt-e shuʿarā as a 'warrant' for naḳhl-e muḥarram :

'How could the tree of the garden equal her glory/appearance,
Even if, like the 'tree of Muharram', it would become from head to foot a sword?'

If this is correct, then through the affinity with a sword, the 'bringing of fruit' becomes even more meaningful.

Atish has well versified the theme of the naḳhl-e mātam , but in his verse the verbosity is greater, so it doesn't have the mood of Mir's:

janāzah ho chukā taiyār ai sarv-e ravāñ apnā
shigūfah phūlnā bāqī rahā hai naḳhl-e mātam kā

[my funeral procession has already become prepared, oh walking cypress
there has remained the flowering and blooming of the 'tree of mourning']

Mir has versified the theme of the present verse once more, in the second divan itself. But in his case too, in that verse there's verbosity and artificialness [{702,7}]:

tābūt pah bhī mere patthar paṛe le jāte
us naḳhl meñ mātam ke kyā ḳhūb ṡamar āyā

[even the stones that had fallen upon my bier, they bore away,
what fine fruit came/appeared upon that tree of mourning!]

FWP:

SETS == WORDPLAY
MOTIFS == [DEAD LOVER SPEAKS]
NAMES
TERMS == MOOD; THEME; WARRANT; WORDPLAY

For discussion of textual problems in this ghazal, see {757,1}. I have used the kulliyat text rather than the one given in SSA.

Could the analogy of stones to the dates borne by a date-palm be part of the verse? The idea of 'fruit' seems to point in that direction (as in {702,7}). In Urdu, the sense of naḳhl seems indeed to include stylized and artificial 'trees' of decorative kinds (see the definition above).

But I keep thinking, through the association with Muharram, that the 'tree of mourning' ought to be analogized to the tall tree-like poles or standards [ʿalam] (see the definition above) that are carried in the Muharram procession. I can't help but feel that they are evoked at least to some extent. By contrast, a bier or coffin [tābūt] (these are also called tāziyah ) seems much less tree-like.

But then, these tabuts or taziyahs are often made extremely tall and vertical, so that they too could be considered to resemble trees. Here's an example, a watercolor c.1830-40 from the British Library collection that shows some very tall and relatively tree-like taziyahs:

The modern Persian naḳhl-e mātam actually has a tree-like shape (with thanks to Zahra Sabri for finding the image):