Geometry doesnt make that suggestion, ballistics (in the technical sense of the word) does.
As far as missiles go, they are absolutely more effective up high than at lower altitudes. At higher altitude, the drag force on a missile is decreased, compared to at lower altitudes. This is because at higher altitudes, the air density is decreased, and drag force is proportional to air density. Double air density, you double drag. In addition to that, at higher altitude, the THRUST of the missile ALSO increases. This one is a little more complex, but the upshot is that in higher pressure environments, more energy is spent moving the high pressure air behind the rocket, out of the way of the plume - whereas in lower pressure environments, at best in a vacuum, that energy is spent on pushing the rocket forwards rather than air sideways.
So as you increase altitude, you decrease the loss of delta-v from aerodynamic drag, and you increase the delta-v available, as your specific impulse rises. The total change in velocity the rocket can achieve is much greater at altitude than it is, on the deck. Its such a big deal that, even if the aircraft is at low altitude, real world missile navigation programs (and the missile navigation program in BMS, lest you think Im knocking them here) will command the missile to climb up above the target, unless the target is already so close that range is not a concern at all. Its worth it to get up into that thinner air.
Missiles dont tend to use fins to turn. The fins are there as control surfaces primarily - the lift to turn is body lift. And for the most part, their speed means even with a low turn rate, the ability of the target to pull g is less relevant than other factors affecting miss distance. Several public papers have been written on the topic of missile miss distances, and best case tactics for defeating missiles inside their effective range. The short version is that missile effective blast radius for a modern missile is much larger than the miss distance its possible to generate solely using aerodynamic maneuvering. If you can introduce errors in missile navigation through electronic counter measures, or if you can defeat the missile kinematically - denying it the range required to reach you - then that is a different story.