The thing is that the car lowered a lot in the back compared to the front and i'm not sure if it sits as it should.
Lowering springs have been around for a long time and I've seen this time and time again going back decades. The spec for this kit does not say what the drops are but it is very common for it to be more at the back than it is at the front. Why? Stupidity.
Kid looks at his car and sees that it sits higher at the back. Ooo, bad. So he buys a lowering kit that drops more at the back to 'fix' it. That's what manufacturers make because that's what buyers want. But every single production cars sits higher at the back and our hero never stops to consider that there might be a reason for that.
If a car is sitting higher at the back it looks ok. If it's sitting lower at the back it looks broken. So manufacturers design the car to be higher at the back while stationary so that when moving with the traction of the wheels pulling the back end down slightly, and with people in the back, the car doesn't look wrong.
So with lowering springs the car looks level when stationary, ye-he! What you don't realize is that on the move with a couple of pals in the back the car looks like a guinea pig dragging it's backside on the ground.
So yeah, it's almost certainly supposed to look like that because that's what most sets of lowering springs do.