Query Performance When Dropping Multiple Columns
Yes, but it's too small to be noticed unless you plan to drop hundreds or even thousands of columns ;) 

Each drop column is executed separately regardless if it's a separate statement, or part of a statement. The performance gain would be in the work postgres do before and after each statement, for instance taking a lock on the table, creating implicit transactions, and parsing the statement which it would only have to do once if all columns are dropped in one statement.