The goal of a cocktail is not to hide the taste of the alcohol. It's to bring together flavours that complement the taste of the alcohol. For example, a Negroni is a cocktail that is made of 3 different types of alcohol - gin, red vermouth, and campari. But even in a more popular drink like a margarita, the citrus flavours lime juice and orange liquer complement the taste of the tequila. If you're just mixing juice with a shitty liquor to get drunk fast then of course the goal is to make the taste less harsh, but once you're no longer in college you can refine your palette and try higher quality stuff.