The worst part of this quote is that, in the original, she (Marilyn Monroe) actually framed her "worst":
>I'm selfish, impatient and a little insecure. I make mistakes, I am out of control and at times hard to handle. But if you can't handle me at my worst, then you sure as hell don't deserve me at my best.
So in the context it sounds more like "here are my flaws - take me or leave me, but you won't change me". Which sounds reasonable. But without that context it sounds more like "I'm entitled because I like to pretend that I'm above other people".
This is a similar thing to "the customer is always right", where the meaning has shifted due to lost context. The original quote was "the customer is always right in matters of taste". Basically, it meant that if the customer wants to buy something, they're not wrong and stupid because the seller thinks it's wrong and stupid to want to buy. Not that the customer is in a perpetual moral high ground over the business and should be granted every wish.
I've seen this before but is that truly the origin? On the Wikipedia page, the quote(s) do not seem to allude to taste or buying preferences at all but rather to customer service. I've tried searching but I haven't seen any primary sources state that the original quote, or intent, was with the inclusion of "in matters of taste".
Okay, I have a sneaking suspicion that it's kind of an apocryphal reverse-explanation to counter currently all-too-common abusive behavior towards service personnel. I think it's just an old motto that once made more sense than it does today when it's been in use for over a hundred years.
It depends, at their worst are they abusive to their friends, family etc for no good reason? If so, then I'd agree it is nonsense used by abusers. If it's said by someone who gets treated awfully for having a rough day, week etc and gets treated badly, well then this quote is true.
I feel like I've never seen or heard of anyone good using that quote. I'm sure it makes some sense if used in genuine good faith. The quote would make sense applied to someone with a disability, for example, by interpreting it more along the lines of having to deal with the person not always being outgoing and maybe even sometimes needing extra help.
But no, I've only ever seen shitty (or at least allegedly shitty) people use that quote, to justify their shittiness. The "worst" they refer to is usually bouts of anger or abuse.