As it happens, I've seen this connection between "maddow" and Somalia once before in a Twitter conversation between Somali men talking about their tastes in women. It's tricky to look up because I'm pretty sure it's an English-ization of a Somali word, so the spelling is a little subjective. As far as I can tell, "maddow" is the word for ethnic Africans in Somalia.
This qualification seems necessary because as far as I can tell, Somalis are very sensative about racial identity. (This is probably what makes the jokes funny to those who know the context.) See, for example, this forum thread:
"Somalis are not black peoples, however we are Africans. I have nothing against maddow people but saying we are related to them is far from the truth. I can see how in western countries anyone with one drop of black is considered black but that's blind ignorance for the most part."
It may be that this particular person has "nothig against maddow people," but Hamdi's jokes help make clear some of the stereotypes. I was able to figure out that hooyo means mother and aabo means father, so many of the jokes are about Somalis seeing maddow children as fatherless.
The part I can't quite figure out is whether maddow just means black or whether it's a specific ethnicity or tribe. Searching around I find that there's someplace called Cumarow Maddow in a region of Somalia called Shabeellaha Hoose.
A brief mention here again implies an association between "maddow" and black: "We crossed some of the highest points of Al-Maddow, The Black Mountain region of the Gulley Range."
Here, in an ethnography collection from Colby College, I find there's a village in southern Somalia called Maddow. (Scroll. It's about mid-page.) But it's hard to believe that this village would become synonymous with a whole class of people.
If you haven't already, try Michael Blackson, referenced above, in the google. He's called the African King of Comedy, and his stand-up often includes jokes about his dark skin (which explains why Hamdi would use him as a reference point in Somali maddow jokes). I thought to link to one of his routines here, but the language and subject matter is a little more than I'm comfortable linking to directly.
Perhaps more than usual, if you know anything about this subject, you know more than I, so I appreciate any further light you can shed.
Also, if maddow is more of a slur than I realize, I sincerely apologize and hope you'll understand I'm only curious and intend no offense.
UPDATE: From Facebook, Omar, a native speaker, has an answer:
As a fan of the Rachel Maddow show! A brilliant show all around... Keep up the great work!! I am happy to lend a little help in linguistics in my native tongue and help clarify the meaning of the the word the Maddow or Madow. It literally means black, the color black...So when someone says Maddow people, one is referring to "black people" ... Somali people call themselves somalis, and not black people, the definition of black people isn't as straight forward sometimes... Black people have varying cultures, languages, and religions. When somali says black people, it just means black people. It can sometimes be a generic label that doesn't do justice in defining everyone who has black skin. So let's just say we are all Maddows lol