Is eating smelly things in the office rude?
You should assume "yes" because by doing this you are disregarding the impact that this has on people that cannot control it and must take action to try to handle it, or you are unaware of the problem it creates for them. Either way, it should be avoided.
The word "rude" means "not showing good manners toward others". Good manners are a way of helping the people around you to be comfortable and acting in such a way that you demonstrate awareness of others. So, by eating anything that has an odor that carries beyond your immediate vicinity in an office, unless you are keenly aware that everyone in smelling distance will enjoy it (or, at a minimum, not be bothered by it), you are being discourteous/rude.
So proceed with caution, since some smells in some offices are "acceptable" while most should simply not be brought into the workplace.
EDIT: The question was changed to:
Is eating smelly things in the office unprofessional?
Again, the answer is "yes" in general. Being a professional means being aware of how your behavior impacts those around you. Professional manners are every bit as important as social manners, especially because people are brought together out of necessity and are usually required to spend far more time in close proximity compared to social situations.
The approach to foods that smell is very similar to music, appropriate lighting, political/religious discussions or any other situation where the activity may cause distraction or offense. With smell (and maybe music for some people) even nausea or physical illness.