I would be excited if someone criticized what I am working on if they were planning on being part of the solution

I'd love to hear thoughtful constructive criticism during an interview. But unsolicited negativity would be a bit of a flag.

Seems like a complete wast of time in an interview. If you join then sure mention it, and if your rejected you can send an email or something but it's not constructive during an interview.

We interview specifically for people who can give and take constructive criticism. There are points in our interview process where giving a rational and constructive critique of our product would be a very good thing for a candidate. These are skills we value highly as they help employees improve faster.

We also do check for negativity in candidates though to balance this. Critical feedback can be given effectively without needing to be rude or unpleasant.

Typically socially well-adjusted people can accept criticism on a project/product without taking it personally

Depends on the context and how it is delivered. If a job applicant started talking about my team's product "being pretty bad", that would be a red flag—I can only imagine what their code-review style would be like.