Learn Italian: Il mio amico domanda se l'aspetto è importante. - My friend asks if appearance is important.

Here, se translates to 'if' or 'whether'. We use it to report a yes/no question (indirect speech). If the direct question was 'Is appearance important?', you report it to someone else by connecting the main verb and the question with se.

Could I use chiede instead of domanda?

Yes! Chiedere and domandare both mean 'to ask' and are highly interchangeable here. Domandare is slightly more focused on the act of asking a literal question, while chiedere can also mean 'to request', but both work perfectly in this sentence.

Why is there an article before aspetto (l'aspetto) if the English translation is just 'appearance'?

In Italian, abstract concepts and general categories always require a definite article. While in English we just say 'appearance', Italian requires 'the appearance' (l'aspetto) when talking about it as a broad, general concept.

Do I need to use the subjunctive mood after se here?

In spoken and everyday Italian, no. When reporting a question like this, using the regular present tense è (the indicative mood) is completely natural. You might occasionally see the subjunctive sia in very formal writing, but it is not required here.