Answer
$\therefore$ Low density makes it harder to generate a lift for the plane to fly.
Work Step by Step
$$\frac{1}{2}\rho v^{2}$$
The above equation is responsible for the effect of fluid speed.
Since, $\rho\downarrow$ and So, $\frac{1}{2}\rho v^{2}\downarrow$
Lift is the result of the $\textbf{Pressure difference}$ between the above and below part of the wing.
So to create such a Lift the plane has to run at greater speed.
$\therefore$ Low density makes it harder to generate a lift for the plane to fly.
