The better your aim is, the higher the impact mental has on your aim/consistency. At the top of the pro level, with aimers like ScreaM, TenZ, Aproto, I would make an uneducated guess that aim is somewhere from 75-90% mental. Your confidence, anxiety level, and your aiming habits.
Your mental stability determines your aim consistency. Here's an anecdotal example. if you go in a deathmatch, and you whiff two easy shots in a row, you'll probably lift your head to the sky, scrunch your eyes, and take an inhale of pure, but brewing, anger. What you're doing in that moment, without even thinking or saying a word, is saying something like "Wow, I am so bad, I can't aim," etc.
This reduces your confidence, and in turn, your focus, because you are creating a source of anxiety for your mind, and as we all probably have experienced, anxiety ruins focus and destroys flow.
Another thing I've found out playing this game, at least in my experience, is that confidence is synonymous with first shot accuracy; the lower your confidence, the lower your FSA and the higher chance you will commit to bad sprays / fights [Of course, different people manifest low confidence differently; some people camp, some people tilt-fight, and some people crouch spray]. This is why you see people like Aproto, for example, just smoothly tap heads all the time; he's clear-headed, focused, and confident, so he retains his good aim habits (crosshair placement, not shooting until on the head), his focus, etc. By the same token, this may also be the reason why you see mechanical-god aimers like Aproto, TenZ, etc. have bad aim games, because the aiming in this game is extremely punishing, especially for not hitting your first shot, and the mental pileup that can happen over the course of two or three rounds can fuck you up for the entire game.

Simply by reacting negatively to a whiff, you are simultaneously telling yourself you're bad at aiming, which then lowers your confidence, clouding your focus and your judgement, preventing you from going into flow, and decreasing your first shot accuracy. For this reason, it's important to create a plan / mindset for yourself to stick to every day so that you can keep your confidence, focus, and therefore aim stable and consistent perpetually. For each aiming situation, create a reaction for yourself to have.

Whiff an easy shot
Positive reaction: Laugh. Laughing means you're confident enough in your aim to know that whiffing is a one-off, and next time, you'll hit the shot.
Negative Reaction: "Oh my god, I'm so bad," "How did I miss that shot?" Lowering your confidence, dwelling on the past and creating negative emotions in your mind, and creating anxiety while you aim and after you lose an aim fight again.

Deathmatch Oper?

Hard challenge, good practice.

Deathmatch Camper?

Have to make sure my crosshair placement isn't lazy.

One Tap?

Nice shot [builds confidence, increases focus].