Almost like dumbing down, or under utilizing yourself?
You are probably looking for self-deprecating:
tending to undervalue oneself and one's abilities.
belittling or undervaluing oneself; excessively modest.
The Free Dictionary
Undersell (as per dictionary.com):
- to advertise with restraint
- understate the merits of something.
If you say something like "Oh, I'm not that smart," you are probably underselling yourself.
You can also humble yourself.
Humble (as per dictionary.com):
- to lower in condition, importance or dignity
Are you asking for "make yourself appear stupid," or "make yourself feel stupid?" In the first case (giving the illusion of stupidity), there are some answers here already that I won't steal. In the second case (feeling embarrassed about your own stupidity), you could try "humiliation," as in "he humiliated himself in front of the whole team."
However, speaking under correction, I don't think that there's a word that covers both meanings at the same time.