Add the oil to a large cast iron skillet or large frying pan over high heat. Add the corn and cook on each side for 5-8 minutes or until the kernels begin to char. Turn each ear of corn to char on all sides.
While the corn is cooking, dice the red onion and jalapeño. Chop the grape tomatoes and cilantro and mince the garlic.
Place the cilantro, garlic, avocado, mayonnaise, lime juice, water, salt, and tajin into a blender. Blend until smooth.
Remove the corn from the cob with a sharp knife and place it into a large bowl. Add in the chopped jalapeño, diced red onion, tomatoes, and cotija cheese. Toss with the dressing and serve.