Long time rat player, have tested a good portion of rat commanders. Karumonix himself isnt especially powerful and especially on a budget (just the nature of non-colony rats being really bad).
Right now, 7 of your 30 creatures are not rats. With only 23 rats, its likely leading to some inconsistency with Karumonix when he ideally should be a semi reliable way to draw cards on ETB.
Another main issue is that from what I can tell, you have close to zero forms of evasion for your rats. Considering how lackluster a lot of rats are, simply being 1/1s with an etb and such, it is unlikely that you'll be hitting enough to kill them with poison. Ideally with someone like karu, youre wanting to go faster than others, and I expect that to be difficult currently with the curve and overall lack of rats/evasion.
For your draw spells, I'm not a fan of the ones that require sacrifice. In a deck like Karu, and considering your current rat count, you don't really have the spare creatures to throw into these spells. I'm also not crazy about stuff like coveted jewel or the eye of vecna. Theyre fighting your curve and are likely to disrupt your early game. In the case of the jewel, considering you have close to no ramp, I doubt that it will be very castable. Id look at a sign in blood at the very least.
There is a lack of any type of ramp in the deck which is probably contributing to some of you challenges.
Cards like bad moon are not very useful for you as you want evasion, not stats. There wont be much of a difference on whether your opponent blocks a 1/1 or a 2/2