How would someone build a physical filter that only works in one direction? I'm having some trouble finding good sources on air filter design, I did see some mentions of differing porosity on different sides looking at HVAC filters but unfortunately no details on the mechanisms/principles behind it. Most of the information I have seen regarding lack of mask effectiveness comes from mask type, improper use, and conservation of supply for those most in need, not the actual mask itself, implying that masks are in fact effective if they are used properly, with effectiveness depending on mask design - your comment appears to be referring to use in general.

I would think however that placing any physical barrier in the path of an airflow is going to have an effect regardless of direction, as some non zero portion of particles are always going to impact the barrier.

Edit: It appears from other comments you may in fact be referring to improper use of masks that are also not specifically designed to maximize filtration ability. I would still submit that some degree of mitigation is better than none, but I can see how a false sense of safety could outweigh the benefits of the mask.