Back to the Future is a veritable jambalaya of movie genres. It's a big part of the reason we find it so irresistibly fascinating. It's a comedy, certainly, but it's also an adventure story, with elements of sci-fi. It has action/thriller bits and pieces, feels like a rom-com at times, and even has some of the same properties of a musical. It's hard to pin down.
More than anything, though, BTTF is a comedy. It may be framed by sci-fi elements, but its main purpose is to make us feel good and evoke laughter…and it delivers. All the time travel and fading photographs are there to lend something unique to the experience, but they don't take away from the fact that, when Marty tells his Uncle "Jailbird" Joey to "get used to those bars, kid," we laugh our butts off.