It's definitely viable. By the rocket equation:
$∆v = v_e \ln \frac {m_0} {m_f}$
Methane-LOX gives an exhaust velocity around 3500m/s. Mars surface to orbit requires the expenditure of about 3800 m/s of ∆v.
So the ratio of initial (fully fueled) mass to final (fuel spent) mass need only be around $e^{\frac {3800} {3500}}$ = $e^{1.09}$ or 3:1 -- 2 tons of propellant per 1 ton of empty ship/payload.
This is as compared to an Earth-launched SSTO, which needs about 9500 m/s, about a 15:1 mass ratio for methalox!