S. R. Faruqi:

a;sar = sign, mark, token.

We have no existence, or else our existence and nonexistence are the same thing-- this is a favorite subject of Ghalib and Mir; or rather, it ought to be considered to be among the traditional themes of Urdu poetry. In the present verse, Mir has versified it in an entirely new manner. We are a face that can be seen in the beloved's mirror-- in that idea there's also an interesting ambiguity. The face that can be seen in the beloved's mirror can also be the beloved's face. If that's the case, then in the first line something is shared between us and the beloved's glory/appearance.

This shared thing can be amazement, because when the beloved's face shows its glory/appearance in the mirror, the image of the beloved's face becomes stupefied and doesn't speak. Thus we are so amazed that no one even perceives a sign/trace of our existence-- well, we're as silent and motionless as an idol. In the light of this meaning, the idiomatic meaning of nah puuchh becomes even more meaningful.

If we are an image in the mirror, the intention can also be that we have the aspect of your mirror-- that is, the way your mirror, from the effect of your glory/appearance, becomes stupefied and still, in that same way we too behave. 'That desert' can refer to the desert of creation, or to the desert of passion. Because of absorption in you, we have become so lost that we are like the image in your mirror, in the desert of passion there's no sign/trace of us. Or else, in that desert we will leave no sign/trace of ourself.

Or, the way that in your mirror the reflected forms keep coming and going, and don't remain-- we too are like that, we have no abode, no address.

A further pleasure is that by giving for himself the simile of something as refined/subtle as a mirror or an image in a mirror, he has also conferred praise on himself.

FWP:

SETS == INEXPRESSIBILITY; KA/KE/KI
MOTIFS == DESERT; LIFE/DEATH; MIRROR
NAMES
TERMS == THEME

But what is it that the addressee is not supposed to ask? The grammar of the question appears a bit unmoored. (When Ghalib uses this structure, he provides a clear indication of what's not to be asked.) If the (rhetorical?) question applies to the first line, then perhaps the idea is that the speaker's identity is unfathomable, incomprehensible, so that he barely exists at all. And if it applies to the second line, then the suggestion is that the addressee might be inclined to look for the speaker in the 'desert of passion', perhaps by locating landmarks or signs or traces of his location there; but is now being warned that such a quest would be in vain.

A further complexity is provided by the idea of the aa))ine kii tim;saal -- does it mean the image or likeness of the mirror itself ('We are like your mirror'), or does it mean the likeness shown in the mirror ('We are like the reflection in your mirror')? Here's one more example of the ways that kaa / ke / kii can be fully as multivalent as an izafat.

It's also hard to know how to hook the two lines together. Is there some metaphoric identity between the 'mirror' and the 'desert'? Or is the connection instead between the 'mirror' with its inert reflectivity, and the lack of sign/effect that characterizes the speaker's life? This verse feels a little too obscure to be truly satisfying. It's the kind of thing people complain of in Ghalib, so it's fun to see the so-called 'innocent' and 'naively emotional' Mir doing it too.

SRF's claim that the ghazal in general (and Mir and Ghalib in particular) take as a favorite theme the question of the lover's existence and/or nonexistence is entirely correct. Here's one of Ghalib's on the subject-- outwardly it bears no resemblance to the present verse, but inwardly it does:

G{210,5}.