Nirvana and Michael Bolton have both sold about 75 million albums total. Nirvana has 25 RIAA platinum certifications, and Michael Bolton has 27 RIAA platinum certifications.
This would put them neck and neck for popularity, but you asked about the 1990's specifically. This puts Nirvana in the lead.
Michael Bolton started his career in the 1970's, and has continued to perform and release new albums to this day. His career did peak in the 1990's, but I'm not sure if most of his albums were sold in the 1990's, or if most of his album sales were spread out over the other 4 decades that he's released albums.
Since most of Nirvana's albums were sold between 1992 and 1995 means they were much more popular than Michael Bolton during those years for sure, but very likely, that Nirvana's sales those years were enough to beat Michael Bolton for the whole decade.