She would use a dinky gun like that.

she is working!! as assasin or mercenary now??

Isn't this a seiyuu joke? I'm not entirely sure.

I think this is a parody of Gunslinger Girls. I'm not sure, I haven't seen that show in a while.

The stance is an obvious parody of Gunslinger Girl's Henrietta (see post #164011).
The depiction of the weapon isn't very good in the first place, but the spent casing ejection is perhaps most troubling. The P90 ejects downward, immediately below the chamber behind the pistol grip. This is not a minor detail, since a non-standard ejection system is mandatory for an ambidextrous bullpup design.