To be fair, there is no official definition of "underbiked". The way it is most commonly used in MTB discussions is a bike with excessive DH capability than is needed for a certain type of riding.
However, it is not synonymous with "too capable" or making things "too easy". Those may be true in some cases depending on one's goals for riding, but it is not really the point.
Being very DH capable, as another poster points out, usually means more travel, longer, and slacker head angles. They also end up being heavier. The issue with this on less technical and/or less steep terrain is not so much that it is too easy or boring, but it can actually be a drawback (at least in the minds of those using the term "overbiked"). A bigger bike is going to be slower on the flats, rolling terrain, and on climbs. It is more work to pedal a heavier bike with more travel. There is a reason you don't see Enduro Bikes in XC races often (and never at the higher levels). Not even in XC races in mountainous "Enduro" terrain. They are slower in these scenarios. In other words, less capable for the task at hand.
Even beyond racing results (which I could care less about 99% of the time), some people simply big bikes less fun when they are not pointed downhill or on rough terrain. They are not as responsive or playful in some scenarios. Less "poppy". So in some ways, they are less capable in that terrain (depending on what you want from your bike).
This is very different than simply being too nice of a bike. A $10K Short travel play bike is going to be more fun for most people than a $1K version of the same type of bike. Also, I don't think anyone would find a top of the line 170mm Enduro Bike to ever be too DH capable and make doing DH runs less fun or boring. Better suspension (for any type of bike) never makes a ride more boring or too easy.
Also, some folks don't even buy the "overbiked" idea. They prefer a big bike regardless of what they are riding, even smooth rolling terrain.
That's my take on this, anyway.