A menstrual cup must be easy to insert and then unfold fully inside the vagina. Only then will it work perfectly and form a proper seal. Pressure from the surrounding muscles must not prevent the menstrual cup from unfolding. A menstrual cup should not remain pressed flat under any circumstances. The material must therefore have a certain amount of tension. Being able to insert and position a menstrual cup quickly and simply is also something that many users really appreciate. This is another reason why a menstrual cup must have enough tension.
However, it goes without saying that the cup should not feel uncomfortably hard for you. A menstrual cup should create no sensation of pressure whatsoever.
When inserted, and provided your cup fits and is in the correct position, you will not feel a thing. You will only feel a sensation of pressure if the cup is not sitting correctly and is too big for you. The degree of softness is not to blame in this case, but the dimensions of the menstrual cup.