This is totally borderline offensive. I think I'd need to know exactly what was said by Seth Meyers to decide because "he also spent some time roasting Jacobs for his outfit before donning it himself." is pretty vague.
I think this light-hearted mockery is pushing on a very specific button that nobody considers a "do not" in the comedy category - cross-dressing issues.
I find that it could be taken offensively because I'm sure there are plenty of anti-androgyny/anti-gay/anti-trans douchebags that find it funny and the fact that a celebrity is publicly mocking this instance of cross-dressing just solidifies it in their mind that it's okay to make fun of or dislike/hate cross-dressers and other people who break the rules of the norm.
I was also watching Fashion Police the other day and one of the hosts mocked Tilda Swinton by saying that she's a man. Although minor, I immediately thought "Oh shut up. Are you really using 1950s gender roles to make fun of Tilda wearing menswear-inspired clothing?"
I understand that Seth Meyers and the host on Fashion Police probably would not have thought of these issues, and that their jokes were all in good fun, but that's almost what makes it disconcerting; it's as if people don't even realize the impact they could have by mocking such a misunderstood minority in the media.
Don't give me the "if he's going to wear a dress in public he should expect to be made fun of" shit either. It's not okay to make fun of somebody who wants to wear clothes that are traditionally designated to the opposite gender.
I'm very much interested in androgyny and the ideas of gender identity, and my style is verging on the point where I'm so bored with what's available for men that I've been considering purchasing women's clothing just so I can wear something new and exciting, so this issue is on my mind a lot lately and I find it very courageous of Marc Jacobs to decide to wear a dress to a prestigious public event like that.