Learn French: Elle sèche beaucoup mieux les cheveux bouclés.

Mieux is the comparative form of the adverb bien (well), so it describes how an action is done. Here, she dries the hair 'better' (mieux). Meilleur is the comparative of the adjective bon (good) and is used to describe nouns (for example, to say 'a better towel').

Can I say très mieux to mean 'much better'?

No, you cannot use très (very) with comparatives like mieux. To say 'much better' or 'a lot better' in French, you must use beaucoup mieux.

Why does sèche have a different accent mark than its infinitive form, sécher?

Sécher is a stem-changing verb. When the ending of the conjugated verb is silent (like the -e in elle sèche), the é changes to an è so the vowel is pronounced openly. You will see this spelling change in the je, tu, il/elle, and ils/elles forms.

Why is 'hair' translated as plural (les cheveux) in this sentence?