In this day and age there are thousands of celebrity Musicians , Actors, Poets, Authors etc. As such a celebrity dies just about every day. At the end of the day it is down to the individuals here to post an R.I.P. on the R.I.P. thread as they see fit.
I think it is reasonable that a legendary figure such as Lemmy from the world of, shall we say none-mainstream music is likely to get attention from members of a forum of a none-mainstream band such as NMA
People mourn other people in relation to how much of an impact they had on them personally, rather than how important the deceased was.