"mostly in response" is correct and can be used in written English.
It can be used to indicate that something was done in response to something else or was mostly a reaction. For example, "He made some changes to the plan, mostly in response to customer feedback."
IAEA's activities in the area of Partitioning and Transmutation (P&T) are mostly in response to the latter.
Those steps are mostly in response to US measures, which in turn were triggered by unrelated actions by the North Koreans.
In truth, this is mostly in response to curiosity from the media and, to a lesser extent, from tourists in search of a golden age of Ireland.
Prices of those securities have fallen 20percentt just this month, mostly in response to a report about anticipated defaults on just two loans.
Savvy eventually reached about half a million women, morphing through various iterations, mostly in response to disbelief in that female buying power.
These were mostly in response to the question 'When you got sick, did you know that other children were sick, too?' I'll read you some of the comments.
These applications mostly come in response to significant news events.
The Department of Buildings does not randomly inspect residences, and mostly acts in response to complaints.
The protests have mostly been in response to China's crackdown in March on antigovernment riots in Tibet and to more general concerns about human rights in China.
A general trend of lipid accumulation, mostly triglycerides, in response to nitrogen deficiency has been observed in several species of different microalgae (Hsieh and Wu [2009]; Yeh and Chang [2011]; Sun et al. [2014]).
While Amazon and Bezos have remained mostly silent in response to frequent Twitter attacks from President Trump, Sanders' recent missives have rattled the company in a way that Trump's have not.
