Are you okay to fly in a plane if you have mild cellulitis?

This shouldn't be a problem. The concern on long flights is immobility of your lower extremities promoting venous stasis and predisposing to blood clots. The focal cellulitis you describe does not sound like it is related to the venous system. As a precaution take frequent walks around the cabin inflight, and while seated work your calf muscles by flexing your toes and forefoot against the ground. Working your leg muscles will help to promote circulation and avoid venous problems.