Should I lower my spironolactone dose from 150mg to 100mg due to potassium level of 5.5?

This needs an in-person evaluation. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

This is more complex than simply yes or no, and a full evaluation is really what's needed. Your age, current medications, diet and health influence this answer. But  one should certainly consider reducing this dose and speaking to your physician. Hyperkalemia (high potassium) is rare with spironolactone use in young women but this should be carefully followed 

If you have blood potassium level elevation while you are on this drug, go back to the doctor who is treating you, not doctors here on the internet

Having a high blood level of Potassium may be due to the spironolactone. You should follow up with both your dermatologist and primary care doctor for further evaluation and treatment. Other conditions also cause high blood levels of potassium.