The idea behind a smart trainer is that it simulates the slope of the road by adjusting the resistance of its brake. All smart trainers and apps work together in the same way: the smart trainer receives your weight and the gradient of the road from the app. Based on the data received, the smart trainer calculates how much resistance it should apply to the brake.
If your smart trainer is not calibrated properly, you have entered the wrong weight of bike and body or you are using the wrong unit of measure, then you could experience your trainer is not simulating the ride properly.
However, if your trainer is calibrated properly, well functioning, and the weight data you have entered is correct, then there are several reasons why you experience that the smart trainer is interpreting the slope wrongly:
You have just started riding indoors, and you are not used to the difficulty of indoor cycling.
You normally ride on Zwift’s platform where the default difficulty is 50% easier than a real ride.
You have picked a mountain route and are not used to the difficulties of riding in the mountains.
You are riding a mountain route, but your gearing is set up for flatter routes.
You can read more in this article about how you can make your ride easier.