What I found in looking back is that most cartoonists eschew their usual comedy altogether, and simply put in some memorial message. Often their characters are seen mourning, and about equally as often, they're seen praising firefighters and sometimes police. I have zero doubt that each and every one of those is 100% sincere. But creatively, there's not much there.
I found three that did something a little different however.
Jerry Scott and Jim Borgman perhaps did the most overtly comedic piece on the tenth anniversary. There's an amusing visual, and the dialogue comes from a character who (within the context of the comic) would likely have been too young to remember the events of 9/11, thus excusing (at least to some degree) his lack of gravitas.
Garry Trudeau took a light jab the Cowboys & Aliens movie that had just come out. As he often does with B.D., he gave him some additional emotional baggage and scarring to deal with. As if having lost his leg in the army during the second Gulf War wasn't enough. The Cowboys & Aliens line isn't a huge joke, but Doonesbury had long ago set aside being a straight-up humor strip and frequently veered into political and social commentary.
Darrin Bell, by his own admission, came up with a very sarcastic approach, giving Lemont a litany of grievances against how our society has changed because of 9/11. He then culminated the strip with the notion that actually addressing any of those concerns is less likely than waking up to find 9/11 was just a bad dream.
I don't doubt that every 9/11 strip is drawn with complete sincerity, but seeing Beetle Bailey (or whomever) cry as he watches the World Trade Center burn doesn't carry much weight to it. It only resonates if you have an emotional connection to the character and, given how flat and without depth most comic strip characters are any more, I have difficulty believing much of anyone beside the creators themselves feel much connection with them. Most strips are written as light entertainment, and there's absolutely nothing wrong with that. But it also means that it's going to make it insanely difficult to successfully switch gears to provide a memorial message that doesn't come across as relatively flat; that's just an inherent by-product of having flat characters.
I was unaware of Bell's work when that strip was first published in 2011; I've only been reading him for maybe a year or two. But the more I see this type of thing, where he's not only being honest with his readers but also unafraid to say unpopular things and to still put a (darkly) humorous spin on them, the more respect I have for him. I read a piece from around that time that called him a weed among the bouqet; I don't know if the author was responding to Bell's notably liberal slant on things, but I think he's reading comics completely backwards.