In 1991, the year "Nevermind" was released, the band Nirvana was not yet nationally known, and nowhere close in popularity to the internationally known super-legendary band they are viewed as now. Who knows if Nirvana would have even reached such levels of fame if Kurt Cobain were still alive today?

Nirvana, before the release of "Nevermind", their first major label release ever, was just a local garage band from Aberdeen, WA (not even from Seattle). Nobody cared about them except some high school kids from Washington state who had seen them live. There was no internet, smart phones, or social media shaping our values and popular culture. People still listened to the radio and bought cassette tapes and CDs at local record stores. The majority of the social standards we are discussing here and now likely didn't even exist as widely shared common values in 1991, over 30 years ago.

Dude wasn't the "Nirvana baby" - he was just the neighbor/ baby of friend of the family who happened to be there at the time. If they didn't use him, any other baby would have been fine. The idea of the photo/ album cover design would have been made into reality either way with or without him as the specific baby.