Add aggregated value filtered by a condition to existing query
Sounds like you want the aggregate FILTER clause:
SELECT time_bucket('1 day'::interval, c.dataora) AS dataora
     , c.codicestazione
     , c.idcomune
     , count(c.id) AS totale_conferimenti
     , sum(c.peso) AS totale_peso_conferimenti
     , sum(c.peso) FILTER (WHERE tiporifiuto = 0) AS totale_peso_conferimento_tipo
FROM   conferimenti c
GROUP  BY 1, c.codicestazione, c.idcomune;
See:

Aggregate columns with additional (distinct) filters
How to count multiple values of a column as one group?