Depending on the gerne, signing to a major label is not a good thing. And at some level the sound gets sacrificed for money.
A mainstream band usually means it was put together in order to make as much money as possible, rather than to actually make music. In other words a commercialized group of individuals, directed at a targeted audience. You can see it in several genres, though most notably being in "pop". I personally dislike mainstream bands, because they're not of the genre I like, or the music they make isn't of the quality I prefer.