Why does Titan have lower surface gravity than the Moon when Titan is more massive? Reading on Wikipedia I saw that Titan is 80% more massive than the earth's moon but has only 85% the surface gravity. Why is this?
Surface gravitational acceleration on an object with mass $M$ and radius $R$ is given by
$$
g = \frac{GM}{R^2} \propto G\rho R
$$
where $\rho \propto M/R^3$ is the density of the object.
If one body has smaller surface $g$ than another, it must have smaller density $\rho$, smaller radius $R$, or both.
Titan is larger than Earth's Moon, so your observation about its surface gravity means Titan must be less dense than the Moon.  Wikipedia confirms:

$R_\text{Titan} = 1.5 R_\text{Moon}$, but
$\rho_\text{Moon} = 3.34\rm\,g/cm^3$ while Titan has only $\rho_\text{Titan} = 1.88\rm\,g/cm^3$.