  We establish an asymptotic formula with a power-saving error of the
$L^2$-norm of Siegel cusp forms of degree 2 in an average sense when restricted
to the imaginary axis. The result is consistent with the Mass Equidistribution
Conjecture for Siegel modular forms and the Lindel\"of Hypothesis for some
twisted Koecher-Maass series. Along the way, we perform a careful analysis of
the Kitaoka formula of degree 2.
