Preparing CREATE statement
DDL queries, including CREATE statements, cannot be prepared.

Moreover, even in DML queries that can be prepared, parameters cannot be used for identifiers. They're allowed at places in the query where a literal would be allowed.

As an alternative, you can safely inject column names into a query with the dedicated function that PL/Python provides for dynamic SQL: plpy.quote_ident(string):

https://www.postgresql.org/docs/current/static/plpython-util.html