Diana is supposed to be unpleasant to watch; a disaster. It was literally the entire point.
I keep seeing posts and comments about her being 'the worst' of Don's affairs (which is a weird way to engage with a show like this, in terms of characters -- none of them are supposed to be paragons of virtue or particularly 'titillating').
But Don's relationships with non-main characters have always served to typify something about Don's character at any given moment. He has the affair with Sylvia as a protracted act of self-injury, and it's meant to be gross and squirmy how Don has "fallen" to the level of sleeping with a married woman who lives near him and with whose husband he has a genuine friendship. Bobbie Barrett was about showing that Don's consistent compartmentalising of his affairs will soon become impossible, and that the women he sleeps with aren't just disappearing into thin air with no agency (it's interesting that she's the first person to let him know of his own 'reputation', and whom Don is the closest to on equal footing with power-wise. It certainly explains why he ends the affair in the midst of a sexual power play).
People complain that the show wastes it's final episodes on a sad mopey waitress but of course that's the point. We want Don to be out there wrapping up his relationships and conflicts with a bow but he chases a ghost across America after literally sleeping with a woman in an alleyway (and paying her for it). His obsession with her makes no sense to us but it also depicts a character whose previous sheen on charisma and desirability is waning -- in just five or ten years Don will be an objectively old man and all of his flirtations will have this air of sadness and desperation. He will be trading on the remnants of his handsomeness and fake affability for the rest of his life; it was a way for the writers show a glimpse into his future. Don will never change inside but the world around him will. Don has no idea he's nearing a series finale and his action reflect that.
If Don is hypothetically alive by 1980, take the discomfort you feel watching him sweatily chase Diana and multiply it by 100. He will be a 50-something man who looks his age if not older seeking the company of the precise kind of women who intrigue him; the few women who will return his affections will be those like Diana who are also broken. Any sense of obligation to his domestic life Don feels in season 7 will be gone as his kids are either independent or in the care of another family, ex-wives dead or bought off.
(If Weiner wasn't essentially persona-non-grata these days I'd be morbidly interested in seeing some sort of one-off sequel which captures the pathetic-ness of a Don in late middle age, but aside from being a cash grab it would also make the entirety of Mad Men much sadder in retrospect.)
So, again, whenever I see things about Diana either being an annoying character or a boring plotline I always wonder how it is that this person would expect the show to wrap up Don?