In Impala, the identifiers are always case-insensitive. i.e., tables named t1 and T1 always refer to the same table, regardless of quote characters. Internally, Impala always folds all specified table and column names to lowercase. This is why the column headers in query output are always displayed in lowercase.
Aliases follow the same rules as identifiers when it comes to case insensitivity.
More information on Cloudera's Impala documentation: