Given that each trophic level of the food chain has a decrease of 90% of available energy, would it be fair to say that 1kg of lettuce has more energy than 1kg of beef? If it's not true, can you explain the reason?
1kg of beef has more energy than 1kg of lettuce but it isn't directly related to the trophic level energy loss.

Given that each level of the food chain has a decrease of 10% of available energy

You're all mixed up here. What the rule is saying is that if you start out with n units of solar energy, you lose 90% of it for every trophic level it passes through; Only 10% passes through as stuff that "stays around" materially (i.e. used to build the structure of the organism rather than being burned away as fuel to keep it alive). That means that it takes about 10x more energy to produce the same amount of edible calories in beef than it does in vegetables and grains, if they are separated by one trophic level.
It's not talking strictly talking about the energy density in food, but instead about how wasteful it is to produce the food containing those edible calories from the point of view of the total solar energy invested. For the same solar energy, you can feed a lot more people calorie-wise with vegetables and grains than beef.
It's kind of like thinking how many vegetables a rabbit eats over the couple years of its life from birth to when the rabbit becomes food for you. How long could you feed yourself on those same vegetables? Weeks maybe? How long could you feed yourself, if you ate the rabbit? A few days at most. You can imagine all those vegetables weigh more than the rabbit, but supply more energy than the rabbit meat, while the same mass of rabbit flesh has more calories than the same mass of vegetable. The rule isn't talking about calorie density of food.
Obviously, there are complicating factors, such as the actual composition of the food, since that determines calories and nutrition (which isn't related to solar energy).
The reason the same mass of beef has more energy is that 10% that sticks around keeps on accumulating. This is also the reason toxins accumulate in animals higher up the food chain. A little plankton might have a tiny bit of mercury in it, but a small fish might eat a billions of plankton. And, then a large fish might eat a thousand small fish. And, you might eat one large fish. If that mercury stuck around in everything that ate it, all that accumulated mercury ends up in you.
