As a front desk officer, is chewing gum while at work unprofessional?

It is definitely.
From employer or company point of view the front desk officer should represent the company in front of customers.
That means all aspects of appearances of the officer should demonstrate that company is on the top of to respect customers.
From customer point of view the front desk officer should be the most professional person.
That's why customers are coming to the front desk.
This professionalism is starting with the behaviour and cloth of front desk people.
Don't forget! The first emotion is always vital because customers have it only once! We don't have any chance to give it again to them.

A front desk officer is a person,who is the recognition of an organisation.
the attitude does matter as it reflects the organisational controls as well.
TO me it is not a matter of professionalism, its simply unethical and etiquette.

It is totally unprofessional.
Front Desk Officer should be presentable, at ease even under pressure and go the extra mile to help each and every client

Yes its! It does shows the person more relaxed rather than active.

Hi Isaac !! Greetings of Peace.
Definitely it is not only unprofessional but also rude to do so.
Front office is the face of any organization and directly reflects the image of the organization.
It is the first impression that a customer gets, so one needs to put the best foot forward in Image and courtesies.
It is the image that makes or breaks the image of the organization.
Hope this helps.
Warm regards, Richard.