I'd say there are more. Courage the Cowardly Dog? Very much in the lovable loser camp. The Eds from Ed, Edd 'n' Eddy also fit, but I suppose you could say that's a Canadian show.
I largely agree with Douglas Adams assessment of the cultural differences. I think it's pretty clear that he is on to something in a general sense. But there are definitely exceptions in my opinion. It's just way too diverse and way too complex a formula to ratchet down in such a narrow way.
Homer maybe the lowest version of a protagonist "loser" tolerable to American viewers, but he still has far too much agency compared to a British loser. "Lisa needs braces" and "Do it for her" are very hero-coded, and would never happen in a universe where the Simpsons are a British family.
Another barometer is American remakes of British shows, where the loser character is given redeeming qualities or circumstances rather than just letting them be the losers they are, such as David Brent vs. Michael Scott in their respective "The Office" roles. I suspect soaked-in-the-wool loser characters don't poll well in American focus groups hired by studios.
Same with Peter Griffin but he is confident and fiercely dominant. He doesn’t feel like a loser.
Even Michael from the office who is a “loser”, has a lot of redeeming qualities like genuine care for his employees, terrific salesman and a position of leadership.
Now that I think about it, that’s probably partially why there’s that old copy pasta about it being a dystopian setting. It lends itself well to the concept.
I think that need and loneliness is also expressed in the show. Which is different from other kids shows which are more cartoony (Dexter's Laboratory, for example, though the adults do make appearances)
I wonder if kids shows nowadays feel a need to include more adults because adults are more likely to be watching.
In the eds, if you'd said "their parents have long been dead and the kids are clinging on to what was left", it'd fit right into the story. Eds had dark and serious moments around the lack of adults.