Learn Spanish (Latin America): Mientras más queratina le pones, más brillo tiene. - The more keratin you put on, the more shine it has.

Yes. In Latin America, mientras más is highly common in everyday speech to mean the more, but cuanto más means the exact same thing and is perfectly correct to use here as well.

Why are the verbs pones and tiene in the indicative mood rather than the subjunctive?

We use the indicative here because the sentence states a general, established fact: whenever you apply more keratin, the hair shines more. If you were talking about a specific future or hypothetical scenario (like the more keratin you put on her hair tomorrow), you would use the subjunctive for the first verb and say Mientras más queratina le pongas...

What is the function of le in le pones?

The indirect object pronoun le translates to to it or on it, referring to the hair (which is implied from the context of using keratin). Because you are putting the keratin onto the hair, the hair acts as the indirect object receiving the action of the verb poner.

Why is there no word for the before más, since the English translation says the more?