I know damn well that this band is a joke but for some strange reason, 99% of the people who listen to it will jump your throat the second you say that, claiming that they're serious musicians with shocking lyrics that have something to say and some really hard music or whatever.
I dislike Rammstein for a couple of reasons. First of all, they get hyped for lyrics that are in essence utter crap. Second of all, the prove my theory that every German band since Can and Kraftwerk that managed to become popular abroad actually sucks. Then, their music adds nothing whatsoever, it's not complicated ot challenging in any way. Substituting a lack of musicial substance with third reich references does not work. And finally, most of their fans are stupid kids who claim to be all hardcore, destroy every pit because they have no idea about the mechanics of a good pit but will be the first ones to call security because you broke their nose by kicking them in the face.
What further fuels my hatred for this band is the fact that the only video they over produced that deserved to be called shocking in a way was the one for Stripped and most Rammstein fans I've talked to don't even get it because they've never heard the name Riefenstahl.
Also, maybe, if they hadn't ripped off 80% of their routine of another German band (you might want to have a look at Oomph)...
I guess what ruins Rammstein for me isn't as much the band itself but the hype and the majority of the people who like it as well as the fact that they are often perceived as a "typical German band" for some reason. They're not.
As for insulting mathcore/rock: The standard insults I usually hear is "it's irritating, it has no structure and no melody" - well, yeah, that's why I listen to it.