A spreadsheet, unlike a Feature Class or a database table, doesn't have an explicit type for each column, and it's not uncommon for the column's data type to be incorrectly inferred, or for a single value in the column to throw off the the way the rest are stored.
Without making any significant changes to your input CSV, just spend some time going through every field in the Field Map section of the tool. This allows you to explicitly state the type for each field in the new feature class.
Alternatively, you could also create the new table first, then use Append to copy the CSV rows into it.
And lastly, if you want to tackle this a different way altogether, you can query the Census API directly and the resulting JSON can be converted directly into a FeatureSet that can then be written to a Feature Classs, merged with other data, etc.
- Josh Carlson
Kendall County GIS