Preheat the grill to medium-high heat.
In a large bowl, mash together the sweet potatoes, onion, garlic, cilantro, and cumin. Season with salt and pepper, to taste.
Form the mixture into 8 equal-sized patties.
Brush the patties with olive oil.
Place the patties on the preheated grill and cook for 4-5 minutes per side, or until golden brown and crispy.
Meanwhile, in a small bowl, mix together the mashed avocado, lime juice, garlic, cilantro, and mayonnaise.
Serve the sweet potato fritters with the Avocado-Cilantro Aioli.