[IMO] , i dont think that "diss" is immature...i just CANT see myself saying "WHOA he just totally disrespected you!!!" cause i find that since they have been used so much and like how many words in the English language get transformed into different meanings..i find that "diss" means more of "insulted" then "disrespected"

so back to my point, i dont think diss is immature, because like it was used by adults and teens and children, but its true, maybe we only hear it now in high schools (cause they sure do say it at my school and it seems totally normal) but i dont think that it makes it immature i think its more of a slang word that people and especially teens find more easy or quick or cool to say..i surely dont say it to be "cool" but i say it cause that's just what comes out..i hear it so much (being in a highschool and everything) that it is just a common-everyday word

and also, i think "Diss" not "Diss on" sounds better..i've never heard "diss on"