Pop quiz -- most diverse cast in a current, nationally syndicated newspaper strip! Go!
Um... er... aa...
My guess is that your first thoughts center on strips like Curtis or Herb & Jamal, which prominently feature Black characters. Except those strips feature almost exclusively Black characters. Which I'm glad are out there, but that doesn't showcase much internal diversity.
Now you do have a number strips like Big Nate and Foxtrot which do have some regular non-white characters mixed in with all the Caucasian ones -- and an inordinate amount of credit to creators Lincoln Peirce and Bill Amend for not only including said characters, but just writing them as characters and not as how Black people are "supposed" to talk/act -- but they also have pretty large casts.
Doonesbury is an interesting case because Gary Trudeau has a number of Asian characters, several gay ones, charaters with a range of disabilities, but very few Black ones that I can recall. But here again, he's got a very large cast so no one gets a spotlight for an extended period. I'd be interested to see how Doonesbury's overall cast make-up compares to American demographic statistics. My offhand guess is that Latinos are under-represented, and the Asian and Black precentages are reversed but that's by no means a quantified study; I'm working off memory on that.
Another example I'll cite is Café con Leche by Charlos Gary. The main couple is an African-American male married to a Latina female. His family shows up pretty regularly, so the strip weighs a little heavier towards an all-Black cast, but he does have some white characters, one of which is gay as well.
I like Watch Your Head by Cory Thomas as well. His cast is primarily Black, but one thing he does that I don't think is being used in any other newspaper strip is that the characters all have different skin tones. Some are darker, some are lighter, which is a more accurate depiction of skin color than the typical chocolate brown that's used pretty much everywhere else.
Adding diversity to your strip isn't adding one single Black character. It's creating an overall cast that reflects what society looks like. And maybe the newspaper funnies page, when taken as a whole, does reflect that. After all, while we don't live in a culture that is strictly segregated, there's still a lot of financially-influenced segregation that's gone on and races do have a tendency to get pocketed into certain areas. So maybe that that same thing happens on the comics page -- with nearly all the Black characters relegated to a small handful of strips -- is in fact a truer reflection of America than any single strip by itself can be by virtue of small casts.
But that still doesn't mean we shouldn't try harder. That finacially-influenced segregation we see in real life isn't exactly an ideal, is it?