it depends what your definition of "underground" is. technically it means you release your music independently, or on an independent label, not a major. so you can be popular and still be underground. for example, duck down is an independent label, they have only sold like 3million records COLLECTIVELY as a whole label. but in the early to mid 90's hip hop was still new and the majors were just starting to get involved so everyone was underground at one point or another and even if you scored a major label deal as an artist you had already paid serious dues in the underground. not like today where artists get major label deals outta fucking no where cuz the label thinks they can market you.
and the most popular hip hop radio show (stretch and bobbito) played both so hip hop heads knew both major artists and underground artists. you'd hear a jay-z track and then a non-phixion track. I mean, most hip hop was played on college radio throughout the country and a college radio dj isnt getting payola to only play major artists, he just played what was hot. awesome time for hip hop