"Reflective of its time" so of a time where being offensive and discriminatory was seen as fine or even cool. The show is more offensive than the first Star Trek that was decades earlier. Being offensive has nothing to do with the time period, if people were fine with it, it doesn't mean that it is fine and not offensive.
And this show was mostly made of jokes targeting a minority or showing horrible behaviours as funny. Seems like enough to call it offensive.
This is simply not true, at least for the vast majority of the show. Any jokes that might be read as punching down are generally being told by characters who are themselves the actual butt of the joke. Example: the episode where Jerry and George keep getting misinterpreted as being a gay couple. The jokes are all built around their embarrassment about the fact, and the punchline is never "lol gay people exist".
I think your defense is true in broad strokes, but Seinfeld definitely parodies LGBT+ folk very directly and repeatedly. The hyper-aggressive hyper-camp gay couple who target Kramer, the worries from Jerry about being seen wearing a fur coat because people might make assumptions about his sexuality, the "female version of Jerry" that George dates and worries about how it's perceived, Jerry's handbag, and a hundred less plot-pivotal jabs which really do add up.
Some of those are pillorying the main characters, saying that the concern from the characters is petty and ridiculous, and others less so. As I see it, they aggregate into what feels to me like punching down. You do still need to view it in the timeframe and social attitudes in which it was written, too, but I don't think it earns total carte blanche. "Tasteless" might be a balanced conclusion.
Jerry going and telling the "stupid immigrant" in front of his apartment what to do with his life, and the guy obviously listens because he's a stupid immigrant who has no clue what he's doing with his life
Jokes about Kramer being assaulted by a pedo as a kid
Gaslighting in most of the relationships of the men
Main characters drugging people secretly
Various sexual assault jokes
And a lot of other things that I'm too lazy to list