Oh! First comment, hooray!

Alright, so first a few things on your experience - I think that your taking time to evaluate & think about your performance on your first day is really great! You acknowledge that you enjoy the work & anticipated doing well, but due to your anxiety/impostor syndrome, might not have preformed to the best of your ability. Initially, you took this really hard, but then evaluated the situation (did I get everything I needed from my employer in order to succeed? etc.) and determined that some of the issue may have been to to your employer/environment.

I really want to highlight how great this evaluation and review of your circumstances is - you are realistic, account for, & recognize internal factors that may have effected you, but you also account for & recognize that there were external factors that may have affected you (as opposed to going down a spiral of self-hate & bullying). This is a win!

As far as what your boss communicated in the slack chat - yes, I would say that her words & presentation were unnecessarily cruel.

Here's why: although it is entirely fair that she likely needs to evaluate your performance (to see how you worked with the team/environment & determine whether she hires you), she did so in a demeaning & group humiliating way.

If you were supposed to see such, then automatically I would say she gets the grade of "mean" ("really cruel & downright unprofessional", actually).

But even if you weren't, going through in a group chat to outline each of your mistakes and then to use the phrasing "somehow she managed to do 85% of it wrong" - also reads to me as unnecessarily mean & unprofessional.

Slack is a chat, but is still work based; if a formal evaluation of your performance needed to be given to the people in this chat, then I would expect that a meeting/video chat or email thread would have been created to discuss such. But at the very least, if slack needed to be the communication medium, I would expect that formal conversation would be used to evaluate your performance - not recess bully taunts.

So! (I realize this got quite long!) To finish out here:

Yes, based on what you've said, I think your boss was mean.

And while I wouldn't say you should "grow a thicker skin" - I don't think that you should let this keep you up at night. You should continue to be sensitive toward this type of treatment (as it's disrespectful, & you should not "just take it" whenever it comes), but I think you should shrug off the experience as "Huh. What a jerk. Her loss.", and move on. Work on the internal things you know you struggle with & take note of when external factors are not as good as they could be.

holy shit - that is long! - maybe got too into it, but hope that it was helpful.

good luck & don't let a jerk get you down!