In most cases, the food offered on flights is bland. Food is prepared before your flight takes off ass it would be challenging to prepare meals due to the air pressure when the aircraft is thousands of feet above sea level.
It is also believed that at about 30,000 feet above sea level, your taste buds and your sense of smell are greatly affected. Your perception of sweetness and saltiness drops when you are in a pressurized cabin.
The dry air and the low pressure cause a decrease in insensitive of your taste buds up to 30%.
Also, the way the meals are prepared and stored affects the taste. Before take-off, pre-made meals are stored and thaw when in air. Before the meals are served, they are preheated in less than ideal conditions, and this also contributes to the not so great taste of the meals.