There are easier ways to enhance Mars's atmospheric pressure, so no, don't use Venus materials. I calculated the energy required to lift a kg of nitrogen — or a kg of anything, for that matter — out of Venus's gravity well, and uphill through the sun's gravity well to Mars. Getting it away from Venus was a small part of the total, but that total was nearly 700 MegaJoules, far more than chemically reacting 1 kg of anything could supply. Lugging stuff up the solar gravity well isn't easy!
But there are other ways. For example, there are many objects flying around in the solar system that contain huge amounts of volatiles, such as comets and some asteroids, and some of those are in orbits that are not as difficult to divert to Mars as lugging up from Venus. The savings in energy is greater than the energy needed to make N2 from the NH3 in, say, a comet.
That said, the amount of energy needed for this undertaking currently is far beyond what we can muster.
So no, for multiple reasons Venus shouldn't worry about somebody swiping its nitrogen!