An induction cooker works with a flat coil setup similar to the one shown below. The diameter of this coil depends on the exact model (in the range of 30 cm). This implies that the field is also extending quite a bit into the air above the cooker.
As the large field can be potentially hazardous (think metal ring on finger) there are relatively strict safety standards for induction cookers. The field of of the coil measured at a horizontal distance of 30 cm from the end of the coil and along the vertical axis (z=0 is the center of the coil) is shown below (the absolute values are not comparable with the field above the center but the profile is certainly comparable). The data is from this study: "B-Field Exposure From Induction Cooking Appliances" by C. Viellard and coworkers.
As you can see the field strength decreases by about an order of magnitude in a vertical distance of 200 mm. A very wobbly pan might be less effective but only if it is warped by more than a few cm.