Whisk together the Dijon mustard, whole-grain mustard, honey and mint in a bowl, and season with salt and pepper. Let sit at room temperature while you prepare the fries.
Put the sweet potatoes in a large pot, cover with water and add 1 tablespoon salt. Bring to a boil and cook until a paring knife inserted into the center of the potatoes meets with some resistance, 10 to 15 minutes depending on size. When cool enough to handle, cut each sweet potato into 6 wedges.
Heat a gas or charcoal grill to medium-high. Brush the potato wedges with oil and season with salt and pepper. Grill until slightly charred and cooked through, 2 to 3 minutes per side. Remove to a platter and serve with the dipping sauce.