Learn Icelandic: Hótelið hefur mikið af tekjum.

In the phrase mikið af (a lot of), the word mikið acts as a fixed block meaning "a large amount". It stays neuter singular regardless of the noun that follows it.

Why is tekjum in the dative case here?

The preposition af (of) always forces the noun that immediately follows it into the dative case. Here, the nominative plural noun tekjur (income) becomes the dative plural tekjum.

Since "income" is an uncountable mass noun in English, why does tekjum look plural?

Some concepts that are uncountable in English are actually plural words in Icelandic! The noun tekjur (income) is almost always used in the plural form. Even though it is a plural word grammatically, it acts as an uncountable mass noun conceptually, which is why we measure it with mikið af.

Why does Hótelið end in -ið?

Hótel is a neuter noun, and -ið is the attached definite article, making it "the hotel". Because it is the subject of the sentence performing the action, it is in the basic nominative case.