You can probably do just fine building tinkerbots for competition without the benefit of CAD, if you have enough time for fine tuning.
But part of vex is about learning real-world engineering skills, and the judges will consider your CAD efforts during their deliberations. My company fabricates wastewater cleaning equipment, and doesn’t build anything without CAD. Example here: Dankest Vex Memes? - #5222 by kmmohn