Next comic, we'll turn out to be completely off base with our talk about age and sexual orientation and he'll mutter "damn Asians, takin' our women."
I'm restating this, but Dora is widely estimated to be 28, and Jim is probably in his late 30s/early 40s. This isn't an especially large gap (certainly not as large as the one in the recently canceled marriage of Rupert Murdoch and Wendi Deng), and they're of similar apparent maturity levels and career positions, so had it happened, theirs would be a pretty low risk relationship for cultural disapproval.
In general, I think the issue with age difference is that it gets conflated with maturity difference because our society is very confused on those points (which isn't helped by the lack of an objective standard for maturity), so people of very different ages are assumed to have very different levels of maturity regardless of evidence to the contrary. Thus people get huffy about a 30 and a 40 year old mature person hooking up, but wouldn't blink at an immature 30 year old and a mature 30 year old together. This also slides downwards into age of consent laws, and people who are above them but still too juvenile, but there be dragons.
I will point to the movie Big with Tom Hanks, which generally failed to be controversial despite having an immature 30 year old and a mature 30 year old together- with the twist that the immature one was actually a 13 year old boy agified via magic (perhaps it would have been more badly received with a gender swap?). It's most remembered for a floor piano duet.