Is the pronoun 'most' used before uncountable nouns?

No, the indefinite pronoun 'most' is used in place of a noun.

The adjective 'most' is used before a noun to describe that noun.

The adjective 'most' is used before both count nouns and uncountable nouns.

Examples:

Most have responded to our inquiry. (the pronoun 'most' takes the place of a noun for an unknown or unnamed number)

The winning team had the most points. (the adjective 'most' describes the count noun 'points')

This compound has the most oxygen. (the adjective 'most' describes the uncountable noun 'oxygen')