Hard-coding values in SQL (complex query)

insert the value into the prices_per_client table directly, so you won't need this left join anymore.

or use a case statement in the select-section:

CREATE VIEW ppc_v AS 
SELECT ppc.*,
  CASE CUSTOMER_NUMBER 
    WHEN 112121 THEN 'WALMART' 
    WHEN 119435 THEN 'SUPERMARKET'
    ELSE NULL 
  END description 
FROM prices_per_client ppc

, create a view and apply it to the query:

SELECT '{"sku":"'||sub.item_code||'","PRICE":"'||sub.item_price||'",PRICES_FOR_CLIENTS:['||listagg('{"group":"'||sub.identifier||'","PRICE":"'||sub.price||'"}',',') WITHIN GROUP (ORDER BY sub.identifier)||']};' AS JSON                                              
FROM (SELECT DECODE(ppc.customer_class_code, 'E', ppc.description, ppc.customer_class_code) AS IDENTIFIER, tpp.item_code, replace(tpp.price, ',', '.') AS ITEM_PRICE, REPLACE(avg(ppc.price), ',', '.') AS PRICE, 
      tpl.request_id, max(tpl.request_id) over (partition by tpp.item_code) as max_request
      FROM table_price_list tpl
      INNER JOIN table_price_product tpp ON tpp.list_header_id = tpl.list_header_id AND tpp.request_id = tpl.request_id
      INNER JOIN ppc_v ppc ON tpp.item_code = ppc.item_code

      WHERE SYSDATE BETWEEN NVL(tpp.start_date_active, SYSDATE) AND NVL(tpp.end_date_active, SYSDATE+1)
      GROUP BY DECODE(ppc.customer_class_code, 'E', ppc.description, ppc.customer_class_code), tpp.item_code, tpp.price, tpl.request_id) sub 
WHERE sub.identifier IS NOT NULL
and sub.request_id = sub.max_request
GROUP BY sub.item_code, sub.item_price;