Even on a cloudy day or a sunny cool day,the cabin of my 2021 Mustang GT is warm to kinda hot ?
Not sure what your question is but if you have a black or dark colored car or interior it is going to be hotter inside. you can get your windows tinted. that should help a little. some people use windshield sunshades. I never tried one but, my BIL has one and he says it helps. my car has a black Alcantara interior. and in Florida it is always hot when I open my car. I open my doors or windows for a minute or so to let the heat out. and then crank the A/C.
Mine is white with factory tint. No real issues.
My last one was red and I tinted the rear and quarter windows. It helped quite a lot.
Do you have auto climate control? Reset it by pushing the ‘auto’ button, then adjust only the temperature control down to where you feel comfortable.
Have a black 2020 Mustang in Florida, 90+ degrees, 90+% humidity, AC will freeze you out in recirculating mode. If warranty is still in effect, take it to your dealer.
If you crank the A/C, what’s the temperature of the air coming out of the vents and what’s the outside temperature at the time?