Arthritis can build up from a head that is not balanced, clumsy shoes can threaten injury, heads that don't offer a wide range of vision or ventilation can cause unnecessary blindness or heat suffocation. No one is going to know this better than your mascot. A head that rests on the shoulders will require your entertainer to move their waist to re-direct their viewing screen. Look for a head that has a helmet inside AND doesn't rest on the shoulders of the performer. Keep in mind that if you have a female performer the head should be tried on by her.
Female necks are significantly shorter.
Speaking from experience, most costume heads choke me because I have a shorter neck. Provide an emergency kit for the performer which includes safety pins, thread and needles, scissors and cloth tape in case something gets ripped or torn while at an event. Everyone has their own personality, and that includes those to step into costumes and deliver lines non-verbally. A sense of timing, a sense of fun, a sense of safety, and a sense to dance are great things for a performer to have. Today there are many liability concerns as well.