Why did Jimmy take his mask off in Independence Day movie?

So we can see the expression on his face?

I think it was instinctive.
Due to the high G-force manoeuvre he was unable to breath, and subconsciously, and erroneously, he believed that he could breathe better if he removed the “obstruction” from his face.

@ragingloli Thanks GA! Please tell me more. Also do you recommend a book on Amazon that goes in more depth in the topic? What should I enter in the Amazon search bar?

Lots of people will do instinctive, irrational things during a panic.
Obstructed breathing causes even infants to react by turning their head, or shaking it back and forth, flailing their hands about their face, and when possible, crying.
These instinctive actions are hard wired in us to protect us from suffocation. Unfortunately, sometimes is has the opposite reaction.
It protects babies in case they get a blanket over their face, or an affectionate pet cat gets too close.
As @loli says, it was his brain wrongly telling him wrong information.
Lots of pilots have made the mistake, and it has been portrayed in other movies and some tv shows.
This is a demonstration I think you will find interesting.

When you wear a mask like that or like an SCBA mask, it makes you very uncomfortable to start with. They grip your face and you have to breathe slightly differently. It can seem like you are not getting quite enough air. In stressful times, you tend to breathe more or harder. That exacerbates the anxiety already being experienced from wearing the mask. The urge to get to a “normal” state and eliminate the anxiety is what drives the person to remove the mask when they shouldn’t. Of course, oftentimes removing the mask makes things worse and full blown panic sets in.

I know from experience with my sleep apnea I’ll have a horrendous nightmare and when I wake my first instinct is to rip off my mask as I feel I can’t breath unfortunately this only sends my body into further panic mode until I can calm myself down enough to replace the mask and breath in my normal rhythm again