Yes to all of this, and I want to highlight another important aspect to this...
Our brains contain a series of neural pathways - ie, neurons that function together. Your brain wants to function efficiently, so neurons that tend to fire together will be connected. This is not the scientific way to describe this obviously, but like, imagine trails in the woods... a trail that is walked over and over will be relatively clear and easy to follow, but if you want to go a different way, you may have to create your own trail, which is harder, and probably won't be easy for others to follow.
Neural pathways are kind of similar. The more times you think or say the same thing, the stronger that neural pathway becomes, and what started out as joking turns into a default setting in your brain.
This is also how positive affirmations work. They aren't this magic trick where you say this affirmation a few times and your life changes dramatically. It's a slow process of literally rewiring your brain. If you think you're stupid, you start telling yourself that you're smart and capable every day. It feels fake at first, but over time, you start to accept it, and your opinion of yourself changes.
But also, if you are calling yourself stupid all the time, you are going to approach life as a stupid person. You are going to think you can't do things before you even try, and it becomes a self-fulfilling prophecy. But if you tell yourself you are smart and you are capable, your confidence increases. You start to try new things, and you learn and you grow. You become smarter because you told yourself you're smart.
It's not a fast process, but it compounds on itself. Your thoughts create your life, because your thoughts become what you say and do, and what you say and do is your life.
I also want to say something about passive suicidal ideation, because I have dealt with that my whole life, and I didn't think it was a big deal, because it was passive. I just figured it's a normal thing to think about, and as long as you aren't making plans, or even thinking seriously about how you would do it, it's fine.
But that's not true. All the research on this topic shows that passive suicidal ideation is very serious, because it can turn into actively suicidal at any time. It's not benign and it's not something that you should ignore or play down.
Passive suicidal ideation is thinking about suicide without making plans. Thinking (or saying) things like, "I want to die", "maybe I should just kill myself", etc are not benign thoughts. They are warning signs, and they shouldn't be ignored. Even if you never transition to being actively suicidal, you're hurting yourself mentally and emotionally, and something is causing that, whether it be a mental illness, or an external situation that is unhealthy for you, etc...
We live in a time where these thoughts and jokes are normalized... I literally made a joke like this yesterday, because it's a hard habit to break. Our brains are hardwired to favour negativity, because it was an evolutionary advantage at one point (ie, being hyper aware of dangers in order to survive), and it's important to recognize that and try not to feed into it as much as possible.