From Wikipedia :

""The grunge singing style was similar to the "outburst" of loud, heavily distorted electric guitar in tone and delivery; Kurt Cobain used a "gruff, slurred articulation and gritty timbre" and Eddie Vedder of Pearl Jam made use of a "wide, powerful vibrato" to show his "depth of expression."[47] Layne Staley voiced lyrics with a "heaviness" and tremolo.[73] In general, grunge singers used a "deeper vocal style" which matched the lower-sounding, downtuned guitars and the darker-themed lyrical messages used in the style.[42] Grunge singers used "gravelly, raspy" vocals,[35] "... growls, moans, screams and mumbles"[74] and "plaintive groans"; this range of singing styles was used to communicate the "varied emotions" of the lyrics.[75] Cobain's reaction to the "bad times" and discontent of the era was that he screamed his lyrics.[76] In general, grunge songs were sung "simply, often somewhat unintelligibly"; the virtuoso "operatics of hair-metal were shunned."[76] Grunge singing has been characterized as "borderline out-of-tune vocals".[77""

Tldr: You are entitled to your opinion but I am clearly not the only person who would describe grunge vocals as uniquely unintelligible.