That's a really complex question, I would say that in the way you phrased the question, no it's not, most of the time... Let me explain.
First, to simplify, you can roughly translate how "economical" doing something in space is to how much energy you need to do it (Bigger ship, more fuel, bigger solar panels, etc).
With that in mind, Bennu has the characteristic that it orbits the Sun the same way the Earth and Mars do, but its orbit is more eliptic, so it intersects Earth's orbit. Bennu Orbit
The Earth needs the good old ~365 days to complete a full revolution around the Sun while Bennu takes ~436 days, so if by any chance both are at the same point in their orbits at the right moment, an encounter is possible, but this would happen once every few decades, and overall they will most likely hardly ever meet.
This is relevant in several ways. The first one is that to get any type of equipment, machines, mining infrastructure, to Bennu, you need to first accelerate them to escape velocity from Earth, put them on an intersect trayectory with Bennu, wherever it happens to be in its orbit, and then decelerate to rendezvous. (Or alternatively, travel several years in a complicated mission profile the same way Osiris Rex did, which is highly impractical) Osiris Rex Mission Profile. So that would make putting the equipment on Bennu more expensive than putting it on the Moon, your best bet would be to wait for the asteroid to pass near the Earth, but that could take years, and you would still need to match Bennu's speed and trajectory for rendezvous. So you find yourself in a scenario with very limited time windows and very long waiting periods. For that reason alone the Moon is a way better option, its way easier to reach.
Now, let's assume that the equipment is already there, mining an asteroid would be way easier due to the low gravity, however you have the issue of maintaining those facilities, again, any spare parts or new machinery will have the same logistc issues as before, which would discourage any investment for such a mission.
But even assuming you have everything in place and working nicely, the original reason to do that, as stated in your question, is traveling to Mars. For such a trip to be feasible, you need the Earth, Bennu and Mars to line up correctly in a very specific way, otherwise a ship would need to chase Bennu, rendezvous with it, and then plot a new trajectory towards Mars from wherever Bennu is. That manoeuvre would be extremely inefficient the vast majority of the time (I suspect all of the time), and would require way more fuel than simply stopping (or launching) at the Moon. Additionaly it would depend on the positions of 3 bodies instead of 2. Since the Moon is inside Earth's gravity well, using the Moon does not add any of that extra complexity.
This is not to say that having a refuel (or refill) station at Bennu is not a good idea. Once a high enough number of spacecrafts are travelling through the solar system on a regular basis, refuelling station on asteroids and some specific and weird orbits would probably become a common thing since then you would have enough people using routes where such a stop becomes a convinient thing to do.
