Are the symptoms of rheumatic fever bad?

They could be. Particularly the ones related to joint swelling and inflammation. It is not unusual for the "arthritic" symptoms to be severe enough to prevent you form bearing weight and even walking. Other symptoms occurring several months later on (chorea) may also be debilitating by producing uncontrolled involuntary movements.
They may be very bad. The major features are inflammation affecting all parts of the heart, migratory arthritis of multiple joints, a rash called erythema marginatum, a nervous disorder called chorea, and subcutaneous nodules (rare), usually on the elbows, knees and over the spine. These are just non-tender pea-sized bumps.