Is cortisone effective in treating cellulitis?

No. Only for the redness or color change. You must use antibiotics.
What's the cause. Cellulitis is basically a visible inflammation (itis) of the skin and soft tissues (red, hot, swollen and tender) . It can be caused by many things including sunburn (radiation), trauma (sprained ankle) or poor vein circulation (stasis dermatitis). It need not be caused by bacteria. If you have repeated episodes of this, get to a wound care or vascular specialist, the diagnoses you have is wrong.
No and yes. Cellulitis is usually considered a infectious process therefore antibiotics are a first line treatment. When warrented steroids are also often used for systemic effects. Steroids may be recommended in situations such as spider bite.