No, incorporeality does not imply invisibility or even difficulty in detection. Imagine for instance a incorporeal floating ball of light. Incorporeal only implies the lack of a fully physical body.
Incorporeality also does not imply etherealness, although the two concepts are similar. Ethereal creatures 'manifest'. Incorporeal creatures are simply here, all the time, albiet in a supernatural way that is alot like not being here. It is not possible for an incorporeal creature to unmanifest and make itself completely immune to physical attacks unless its description specifically says that it can.
Note that Ethereal does not imply 'invisibility' either except in the sense of unseen. Unmanifested ethereal creatures are neither here nor invisible. An unmanifested ghost is on the ethereal plane 'looking in' as it were. It cannot be harmed by things that cannot reach into the ethereal, and cannot be seen by things that cannot see into the ethereal. Of course, this distinction is probably over fine, since the spell 'See Invisibility' allows you to see ethereal creatures as well as invisible ones (although it does not reveal which previously unseen objects are invisible and which are ethereal).