It depends what you're looking for. Plyometrics will add speed and explosive power. Yoga will add stability even when you're in odd positions, which can be important when you're in a deep lunge and your opponent either parried or retreated beyond your reach. For foil, the only muscles you actually need to be well-developed are in the legs (particularly the thighs) and the rear, though your posture will be better if you've got some strength in the core and the back. But, honestly, back when I was doing foil in college, I was limited primarily by technique, not fitness. Drilling with a partner to learn things like how to keep the optimal distance away from my opponent, to condition myself to riposte instead of merely parrying (which I found much more instinctive), and recognizing the shifts in stance that indicated different types of attacks, feints, and defensive postures and then trying to adjust my own technique in response were much more effective at making me a better fencer.
The most effective distance drill was actually probably the simplest. You and a partner stand in fighting posture with your weapon arm extended forward. Each of you places a palm on a helmet which is suspended by you both applying pressure. One of you leads for a time, mixing up fast and slow, forward and backward, large and small steps, and the other follows trying to keep distance constant enough that neither of you has to bend an elbow and yet the helmet doesn't drop. Then switch.
The half hour of footwork and lunging in practice was enough to get my muscles in the necessary condition. And this was years before I ever went into a weight room, or even ran regularly. At the highest levels, almost all sports become skill sports instead of merely fitness sports--the best runners are separated by merely good runners largely based on things like efficiency of stride. But in fencing, the primary issue is skill at all levels, from novice to Olympic.