  Using $2.93\,\rm fb^{-1}$ of $e^+e^-$ collision data collected at a
center-of-mass energy of 3.773\,GeV with the BESIII detector, we present a
measurement of the branching fraction of the doubly Cabibbo-suppressed (DCS)
decay $D^0\to K^+\pi^-\pi^0$ and a search for the DCS decay $D^0\to
K^+\pi^-\pi^0\pi^0$. The branching fraction of $D^0\to K^+\pi^-\pi^0$ is
determined to be $[3.13^{+0.60}_{-0.56}({\rm stat}) \pm 0.09({\rm syst})]
\times 10^{-4}$. No signal is observed for $D^0\to K^+\pi^-\pi^0\pi^0$ and an
upper limit of $3.6 \times 10^{-4}$ is set on the branching fraction at the
90\% C.L. We combine these results with the world-average branching fractions
of their counterpart Cabibbo-favored decays to determine the ratios of the
doubly Cabibbo-suppressed over the Cabibbo-favored branching fractions,
${\mathcal B}(D^0\to K^+\pi^-\pi^0)/{\mathcal B}(D^0\to K^-\pi^+\pi^0)=(0.22\pm
0.04)\%$~and ${\mathcal B}(D^0\to K^+\pi^-\pi^0\pi^0)/{\mathcal B}(D^0\to
K^-\pi^+\pi^0\pi^0)<0.40\%$ at the 90\% C.L., which correspond to $(0.75\pm
0.14)\tan^{4} \theta_C$~and $1.37\times \tan^{4} \theta_C$, respectively, where
$\theta_C$ is the Cabibbo angle.
