It's a very difficult balance to strike imo. People do take niceness and humor as signs that you're not quite as "professional". Of course, other people don't make this mistake, but we don't live in a vacuum - sometimes the jellybrains have control over our promotions.

The difficulty is why it requires intelligence to achieve. It is easy to be mean, and easy to be kind to your own detriment. Being kind while still thriving yourself takes thought.

That's because niceness and humor are often just a mask for being unsure, inconcise, or at worst plain unkind. Being kind is much harder, it requires thoroughly judging the situation, including considering own interests, and then responding in a genuine manner.

I think people are confusing what kindness means here.

It’s not about not protecting yourself against abuse but rather not taking advantage of people.

Being kind doesn’t mean you can’t compete or strategize but rather don’t cheat if you do.

Compassion and acts of charity is kindness.