"La peinture, c'est son dada" signifie que peindre est l'occupation favorite de la personne dont on parle.
On utilise ce mot pour parler d'une occupation à laquelle la personne concernée tient vraiment beaucoup.
Un synonyme moins familier : "son violon d'Ingres" (Ingres was a painter but he really played well violin (If I well remembered he had long hesitated between painting and music and his detractors said he should have chosen violin).
On dit aussi beaucoup "hobby" même en français !
Hmmm.. Sorry, I just did a quick check on 'hobbyhorse'. Looks like you can use 'hobbyhorse' as well as 'pet subject'... but in my opinion both are old fashioned... I can't remember the last time I read or heard these expressions being used.