I don't get it. I've heard this kind of comment before, and what is comes down to is complaining that the music in a musical sounds like it is, well, from a musical.
I like songs which have a melody, lyrics which rhyme, and words which fit the music. And that's pretty much a general definition of show tunes. If that's not your kind of music, maybe you shouldn't be reviewing musicals.
I should add, in the spirit of The Whole Truth™, that I have played in concert bands, jazz bands, classical groups, marching bands, a sinfonietta which recorded new compositions, musical show orchestras, stage bands (40's swing), and ethnomusicology groups (African, south American & Asian music). I like all kinds of music. I sing and write pop, rock, country, and sing operettas and musicals and swing and jazz. The only music-like genres I don't like are rap and metal. Rap because it's a very very old Caribbean art form which has had the lilt and humor sucked out of it, and metal because on an oscilloscope it looks like noise.