Caricatural distortion does not equal racism. Many cartoonists make conscious efforts to avoid racism, and sometimes minority group depictions are simply avoided for this reason.
Close-mindedness in political beliefs may have lead to a few of the survey respondents' interpretations, although it is hard to conclude without a more broad and statistically sound survey. However, several cartoons were interpreted in a way likely not intended by the cartoonist, in order to line up with the political beliefs of the respondent. In fact, a few respondents even declined to comment on cartoons that they disagreed with. Therefore I would suggest a reader to always consider the cartoon from an objective or even opposing viewpoint to determine the intended interpretation.
Consider several aspects of the cartoon (for example coloring and references, or text and caricature) before deciding on an interpretation. It is likely that the artist is communicating his/her meaning through many different techniques. If there seem to be conflicting meanings in one cartoon, consider that a misinterpretation of one or more techniques is possible (a complex or dual meaning intended by the artist may also be possible).
For Cartoonists
Pop-culture references add meaning and allow readers to relate to a cartoon when the reference is understood and effectively communicated, but if the reference is vague or not culturally relevant, the cartoon may be misinterpreted. Too many references in one cartoon may cause confusion.
When referencing political events, be careful that over-exaggeration cannot be misinterpreted. For example, many respondents felt that the Glenn McCoy cartoon was critiquing political lying in general, when McCoy was likely taking a straight shot at Obama. From responses, I gathered that the main reason for this misinterpretation was the extreme exaggerations being attributed to Obama.
Titles can emphasize the correct interpretation or complicate and potentially confuse some readers. Concise titles that get right to the point of the critique should be used if the main goal of the cartoon is correct interpretation.
Because readers are subject to misinterpretation, it is wise of an artist to consider the oppositions' view of the cartoon to predict misinterpretations that could be damaging to his/her point.