Mi cabello está mojado.

Spanish has two verbs for “to be”: ser and estar.
• ser (es) describes permanent or inherent traits (e.g. “Él es alto”).
• estar (está) describes temporary states or conditions (e.g. “La ventana está abierta”).
Since being wet is a temporary condition, we say está mojado rather than es mojado.

In Spanish, most singular nouns need a determiner (definite, indefinite or possessive).
• mi indicates possession (“my”).
• Without it, “Cabello está mojado” is ungrammatical.
You could say *el cabello está mojado (“the hair is wet”), but to specify your hair you need mi.

• cabello is more formal or literary and refers specifically to the hair on your head.
• pelo is more general/informal and can mean any body hair (or even fur).
In Latin America, pelo is far more common in everyday conversation; cabello sounds a bit more elevated or technical.

The default word order for descriptive adjectives in Spanish is noun + adjective.
• cabello mojado follows the neutral pattern.
Putting the adjective before the noun (e.g. el mojado cabello) is grammatically possible but rare and often poetic or emphatic.

Yes. Spanish adjectives agree with the noun they modify:
• cabello is masculine singular → mojado (masc. sing.)
• Example of another agreement: mis camisas están mojadas (fem. pl. mojadas).

They overlap but aren’t identical:
• húmedo = damp, slightly moist (e.g. “El clima está húmedo”).
• mojado = wet, soaked or waterlogged (e.g. “Salí de la lluvia y estoy mojado”).
Use húmedo for mild moisture and mojado when something is actually wet.

• mojarse emphasizes the action of getting wet: “I wet my hair” or “I got my hair wet.”
• mi cabello está mojado emphasizes the resulting state: “My hair is wet.”
To talk about the current condition, Spanish prefers estar + past participle; to talk about the action, you can use the reflexive form.