Is the sweep angle of an inverted wing zero degrees? Is the sweep angle of an inverted wing zero degrees, when the angle between the fuselage and the inverted delta wing is 90 degrees? Or is it different because the span of the wing isn't constant over the chord.
Sweep is normally given as the quarter-chord sweep. If not, it must be made clear by some qualifier, like leading-edge sweep.

Your previous questions let me suspect that you are into a special version of ground-effect vehicles, the Lippisch aerofoil. The picture below shows the X-114.



The wing is a delta with negative sweep, so it has a straight leading edge and a forward-swept trailing edge. The idea is to have a positive angle of attack and very little space between the trailing edge and the ground for maximum ground effect.

Such a wing has a negative sweep angle. If you take the weighed average of leading ($\varphi_{le}$) and trailing edge sweep angle ($\varphi_{te}$), you get the quarter chord sweep angle ($\varphi_{25}$): $$\varphi_{25} = \frac{\varphi_{te} + 3\cdot\varphi_{le}}{4}$$