Depends on what you mean by "dangerous". Tornados have strong enough winds to deflect any aircraft and tear most aircraft apart, even in the air. On the ground, tornados are destructive because they pick up objects and fling them around at very high speed. If you were a bird or something else relatively small, I'm not sure whether it would be deadlier being near the ground and at risk of being struck by flying debris, or in the air and if the winds would be strong enough to rip your wings off (it probably depends on the tornado - some tornados are stronger than others).