All hardware has some distortion due to interference or materials used, meaning that the source sounds don’t come out as intended. For example, the cover on the back of the speaker vibrates slightly on certain tones causing that tone to come out different.
Flat frequency response means that the sound that goes in comes out exactly the same. Using a highly sensitive microphone combined with special software you can measure the distortion and set your equalizer to achieve this.
Studio headsets are built to have as little distortion as possible out-of-the-box. Some gaming headsets do good as well.
As for the original question, I’d say it’s probably better than nothing. But still an undesirable situation. Then again, if lousy quality makes the subliminal sound like nails on a chalkboard to your subconscious, I can imagine headaches are a possibility.