Nearly every society has someone claiming that the government must be overthrown, that the status quo must be changed, and on and on... This is the viewpoint of Clan Jamcreermin, whose viewpoints are actually quite correct.
While information concerning this clan is scarce, their origin may have been in the brief (but commendable) heroics of the ex-Skavenslave Skabbicus. He had become a warrior and promised a better life. Against all odds, his revolution held firm, with his fellow slaves being able to break even Stormvermin apart. It's likely they could've succeeded in their endeavors, had the Council of Thirteen simply surrendered. However, the Council was clever, and announced that, if their leader was pointed out, any slave who ceased and desisted would be given a pardon for their misdeeds. Over 10,000 slaves quickly pointed out their leader, causing the rebellion to fall apart (the reverse of Spartacus, if you didn't get it). Skabbicus died that day, and all slaves who had helped in the revolt were killed (the pardon being regarded as hearsay).
If this is the case- if Clan Jamcreermin is trying to save their own species from themselves -then they may be one of the only good Skaven clans. However, that is a considerable amount of trust to place. Even so, if this were truth, the Council would've likely crushed them by now. This does leave one to wonder- is Jamcreermin really saving the day, or are they merely puppets of the Council? Are these Skaven a threat, or is the Council going soft? Regardless, these Skaven are true oddities in their world. They also had their Warlord killed THIRTY-SEVEN TIMES. Due to Chaos reasons he has not properly died, and is possibly hanging out with Kaldor Draigo in the warp. They aren't any of them in Clan Verminus, which makes a lot of sense as no sensible other Skaven would let them there in the first place. Which seems to be fine for Gee-dubs as they only mentioned them twice.
The name "Jamcreermin" is, given GW's continued love of puns and British slang, possibly based off of the term "Jacobin", originally a French term for a member of a revolution-era French political movement that wanted greater centralization and republicanism (that is, wanting a republic instead of a monarchy) but was turned by the British into a pejorative for left wing radicals. That, or its nonsense gibberish, which is also a very British thing.