Okay, so wear some thing perfect, or go barefaced on a packed subway.
Which would you choose, as a civilian without a perfect N-95 mask?
You raise a great point. The thing about PPE- especially respirators- is that any flaw in the fit (i.e. wearing the wrong size) or an imperfect seal defeats the whole purpose of wearing one in the first place.
N95 respirators come in different sizes. Wearing the wrong size for your face distorts the mask. Distorted masks will not "seal" against your face correctly. And if you have a beard- even coarse stubble- the mask will not work.
Some have recommended applying Vaseline to the face to create a better "seal." Nope. Don't do that. The vaseline will act as a "flytrap" for particles- including viruses- that are in the air. You touch it, you get it.
As to your question: if you go into any public place with a poor-fitting or inappropriate mask (like those cheapo blue single rubber band masks that you find in any store), you may as well wear nothing at all. It won't protect you, even a little. Any gap, or the wrong type of filtration, is all it takes.
And please, don't wear a bandana or scarf over your face. Those won't work either and you may be mistaken for an antifa punk or cattle rustler.
The best things to do if you have to go out and don't have the right PPE is to watch your social distance, turn away from anyone who coughs or sneezes, wash your hands, cover you own cough or sneeze, don't touch your face, and avoid non-essential trips for the time being.
And if you have any cold or flu-like symptoms, then for God's sake stay home.
Hope this helps.