Is "mainstream pop music" just a genre or genres?
Whatever it means, can I use it as below?
What kind of mainstream pop music do you like?
What kinds of mainstream pop music do you like?
There is not one universally agreed upon taxonomy of musical genres (though there have been efforts to create one). Most taxonomies recognize that musical genres can be classified in a hierarchical system. For example, the digital music store at Amazon.com has a top level genre (or "root genre") called Pop, and within that there are eleven sub-genres, including Dance Pop, Soft Pop, Euro Pop, Pop Rock, and Adult Alternative.
"Mainstream Pop Music" probably equates to Pop Rock, by Amazon's taxonomy.
To answer your question, music is a mass noun. Unlike count nouns, mass nouns have no plural form and cannot by used with numbers.
And yes, you can use either kind or kinds with music.
But here is another possible way to formulate your sentence:
What sub-genres of Pop Music do you like?