It means "just" or "only".

Overall, it means: Because most mass media outlets are owned by corporations, all of their advertisements look the same. Small businesses aren't owned by large corporations, and so their advertisements are different. People like this because they are tired of seeing the same thing over and over. I'm going to split the sentence into three bits to try and explain it. "Mass media is but a small thrall of it's corporate masters" == is basically saying that most mass media (i.e: CNN, Fox News, The Guardian) is a slave to corporations. Here a "thrall" means being under someone's power, coming from the Old English word for slave. "Slick homogeneity of most advertising." == Homogeneity meaning they all resemble each other. Because they're all controlled by corporations, most mass media advertisements look the same. "The amateurish eccentricities of commercial for local businesses provide a tonic [for this]" == eccentricities means things that make one different from another (typically in a strange way). "Providing a tonic" means that it acts as a remedy. So the advertisements of small businesses will be different from the advertisements of mass media, and people like that.

Nothing more than