If you're just selling clothes online as a side income then you shouldn't need a business license to sell the clothes online.
However if you want to make a full time income from selling clothes online then you'll likely need an office or other place where you can conduct the business and get a business license.
Having the business license doesn't cost that much and they're not hard to get and it would make your business more trusted by customers so it can be a good thing to get a business license if you want to see your clothes selling business succeed.