Certainly not my experience. Most restaurants are mediocre or worse. User review sites are of course highly imperfect and sometimes there just aren't very good options off the beaten track.
But I must say I have generally better experiences with doing at least a modicum of research (whether online, local recommendation, looking at menus, etc.) that just walking in somewhere at random.
Without knowing you and what you look for in food, and in eating out, your expectations of service, your enjoyment of the aesthetic, your appreciation of arbitrary culture, it’s very hard to give this statement any weight. There are more restaurants on my block in SF than my home town, and of much higher quality. If you find the MAJORITY of restaurants to be mediocre or worse, that reveals a value system incompatible with mine. I’m just looking for a new place to eat, not the next Michelin star restaurant. Abundance of choice is my problem that random choice solves neatly.
But yes--I do find if I walk into the random restaurant on a street I expect to be unimpressed, even in SF. And the fact that it's somewhere I haven't tried before isn't really a big plus in my book unless it's also good.
I am at least somewhat picky however.
My sincere condolences.