The structure のが下手 is used to express that someone is bad at or poor at doing something. The particle の nominalizes the verb (turns it into a noun phrase), が marks it as the subject, and へた下手 (へた) means "unskilled" or "poor at."
The verb before の is always in its plain (dictionary) form, regardless of whether the overall sentence is polite or casual.
When talking about your own lack of skill, using 下手 is natural and sounds humble. However, when talking about someone else directly to their face, it can sound rude. In such cases, speakers often soften the expression or avoid it altogether.
The opposite of 下手 is じょうず上手 (じょうず, "good at / skilled at"), which uses the same structure:
下手 is a な-adjective, so it conjugates accordingly: