Is it possible for a 'normal' user possible to find out which mod has deleted comments on a particular question or answer
At least, I'm unaware of any such facility available to 'normal' users.
and is it reasonable to ask for an explanation?
AIUI the primary purpose of comments in SE websites is to assist the author of a Q or A to improve their Q or A. If, after a short while, the author then chooses not to change what they wrote, the comment serves no further purpose.
The author of a comment always has the option of editing the Q or A themselves (admittedly only if they have enough rep). They also have the option of adding another answer - which can reference another answer if need be.
Consequently, I think it reasonable to consider comments as completely disposable. If anything, more comments should be deleted to help avoid people attaching too much importance to them. Maybe they should auto-implode after some period! :-)
It is of course very unpleasant to feel that your contributions have been singled out for deletion in some vindictive way. In the case of comments, I doubt the value to the site of increasing the opportunities for deletee and deleter to wind each other up.