Il y'aura du beau temps demain?
Correct + Il fera beau demain ? Il fera beau temps demain ?
Is there anything wrong with "Le temps fera beau demain?"
yes it is wrong: "le temps sera beau demain"
I thought you were supposed to use faire when you were talking about weather
You are right but only with the phrase "il fait...", not "le temps fait..."
Does "beau" can also meaning " nice weather"? Is it a phrase? Or idiomatic?
Yes, when it comes to the weather, there are several phrases with verb "faire": il fait beau, il fait mauvais, il fait chaud, il fait froid, il fait humide, il fait sec...