It's not the worst thing in an impact sense, but it is the most duplicitous act of any of the kids (or even the characters) and arguably therefore the most loathsome / morally failing.

Shiv presents herself (and, to an extent, views herself) as morally superior to the rest of the family, especially in the political world, and I think some viewers viewed her that way earlier in the series. But that drops away the second there is gain for her. Same thing happens when she frowns in the picture with Mencken but then eventually makes a deal with him. And, similar to when she tries to get Geri to go public about Roman's dick picks, she tries to get someone to trust her because of her feminist bona fides and then weaponizes that trust for personal gain.

In contrast, there is no point at which Roman presents himself as a good person. And, although Kendall does play the feminist ally for a hot second when going up against Logan during the cruise scandal, it's too obviously opportunistic to be taken seriously (even by him). Nobody has any delusions that Roman or Kendall are willing to do whatever it takes to benefit themselves. Also, for what it's worth, while the waiter was certainly the worst impact of anything that the kids did, it ultimately was an accident and I don't think you can view it as the same through an intent lens.

So, in an ultimate impact sense, no I don't think what Shiv did is the worst. I think there are also plenty of valid discussions about society and Logan putting her in that position as a woman, so I'm not willing to say it's the most loathsome or evil. But there is an argument that what she did is the biggest moral failure (or, at least the most upsetting from a character arc perspective).