No, "introvertism" is not a proper word. The correct term to refer to the personality trait of being an introvert is simply "introversion." The suffix "-ism" is used to form nouns denoting a specific ideology, practice, or condition. However, in the case of introversion, the term is already a well-established noun on its own and does not require the addition of "-ism."
So, it's more accurate to say "introversion" when discussing the characteristic of being an introvert, rather than using the non-standard term "introvertism."