True, some of them are maybe not entirely appropriate - but then, when
exactly are the books based? There is no set time for them, and the language over the thousands of years Tolkien's works run over never actually seems to change - maybe there are specific elven insults. But I don't speak elvish, and neither do the majority of players on the game Imladris and I are running - sure, elvish insults may be more appropriate, and could be more amusing to an elf...but we aren't elves. An RPG needs a touch of humour here and there - the insults exchange with the dwarves was simply a way of creating one of these 'touches'
