Variable PWM

You tell us, what relationship do you want between the joystick input and the motor output.
 

A table lookup could work, but your resolution would be limited by table size.  If that's OK (you don't necessarily NEED a ton of resolution, it depends on the application), then you can absolutely go for a table lookup method.  It's fast to look something up in a table, but can be annoying to put a large lookup table together manually.
 
You can make the relationship between the ADC value and the PWM DC any mathematical relationship you want.  If you want a simple scale from one range to another this map macro may be of use.

#define map(x,in_min,in_max,out_min,out_max) ((x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min)

 
So for something like a 10-bit ADC and a PWM with a period value of 5000:

#define MIN_ADC_VAL 0
#define MAX_ADC_VAL 1024
#define MIN_PWM_DC 0
#define MAX_PWM_DC 5000
uint16_t pwm_val,adc_val;
adc_val = Read_ADC();
pwm_val = map(adc_val,MIN_ADC_VAL,MAX_ADC_VAL,MIN_PWM_DC,MAX_PWM_DC);

 
This would scale your input value to an equal point on your output range.

It can be as simple as the ADC reading is the duty cycle (both are 10 bits); you don't need a table or any math at all.  Be aware that if you want full 10-bit resolution in the duty cycle, you have to select a frequency that supports that.