I think @Shokhet is probably correct in that it's likely a form of teasing/light antagonism, but, on further reflection, it probably goes deeper than that.
It is an older, less known name for Ishtar.
Names have power, and Morpheus is showing some by reminding her he knows her entire history. (The Endless are older than the gods, and likely more powerful.)
It is related to Tammuz.
This has multiple implications with negative emotional resonance:
Tammuz is the slain lover of Ishtar/Belili, and so the reference has negative emotional resonance.
When Ishtar returns from the underworld, she finds Tammuz/Dumuzid on his throne, "not mourning her at all".
In the Tammuz version of her descent to the underworld, Ishtar's agency could be said to be diminished by the involvement of Tammuz as the objective—Gaiman is post-modern so this may be a consideration.
In the Babylonian version, Belili is distinct from Ishtar, presented as a version of Geshtinanna, sister (and lover?) of Tammuz, who takes Ishtar/Inanna's place in the underworld.
Belili is the underworld version of Ishtar, and the Sumerian underworld is no kind of fun.
- Using Belili carries the implication that she is in a kind of hell.
Specifically, working in a strip club where dancers have to put up with a lot of crap, as men in such settings do not always act in a respectful manner. This is particularly problematic when you remove the "temple" context [see note], resulting in a degraded form. Sex workers are widely stigmatized.
Further, as I seem to recall, she cannot really dance, in the sense of her status as the Great Goddess, which must be a kind of torture. (She cannot be herself, only a diminished reflection of herself.)
This may be a reference to a certain conception of the cycle of Greek Mythology, which from certain perspectives (certainly Graves's) can be understood as a subjugation of feminine power, which is to say diminishment of the power and status of the Great Goddess.
Note: The idea of "temple prostitution" in Mesopotamia is disputed, but the idea caught hold, as it can be regarded from many perspectives, including a feminist perspective, and is definitely present in modern, speculative fiction. It is this idea Gaiman is referencing with Ishtar's situation and existence. Taking the life of her lovers is sometimes referred to as the Great Marriage (aka death). Gene Wolfe wrote about this aspect of the Goddess in There Are Doors.