It is the opinion of the Hanafi scholars that when the Quran is being recited one must not speak or perform any actions, but listen attentively.

Therfore to kiss one`s thumbs or say SubhanAllah whilst the quran is being recited would contradict this. Allah has commanded that;

“And when the Quran is being recited listen to it attentively and remain silient, so that you may be shown mercy (7:204)

Imam Ahmed Raza Khan (May Allah shower His mercy upon him) says that there are three times when it is not allowed to kiss the thumbs;

1. When the Quran is being recited

2. When the Friday Khutba is being read the audience must listen to attentively and

3. Whilst the person is in Salaah.
(Fatawa-e- Radhwiyyah)

Of course Allah knows best
[Answer provided by: Muhammad Salim Ghisa]