I like to think all my decisions through. Impulse decisions, to me, always turn out wrong.
Underground is usually things that push away from the norm and try to do something different. It is not mainstream because it's not part of, well, for lack of better words the main stream of interest.
There are several underground/indie/what have you bands that make it past the 'rock' and push into mainstream. Turning mainstream can really make or break a band, even more so in the view of older fans. Most bands change their sound and some what sell out so that the majority of people like their music. This = more $$$. Take the EPs or what have you from your favourite band. I'm almost 90% sure they were 'underground' for the longest of time and their sound is totally different from what they are now. That's how all bands start out. Sometimes pushing into the mainstream is good, sometimes it's not.
Mainstream is usually what you hear on the radio. Unless it's a station directed to underground type stuff only.
As for Sublime, they had a one-hit wonder back in the 90s "What I Got" Why? Because its style was "normal" To me, they are ska-like. So, I would say they aren't really mainstream because ska isn't widely accepted. Ska is different.
It's like the band Nightwish. They are popular within the symphonic metal scene and now starting to push out of that scene because of their change in style. Their new singer's pop-like voice and their weaker lyrics have done this. They are a little more widely accepted, but 90% of the population would listen to Sublime over Nightwish because Nightwish is still too different from the mainstream style.