It doesn't bother me. I may not like some of the nns on girls (
Charlie for example) but I'm not bothered. It's their choice and probably just as legitimate as boys using the same nn. It's better than just being a
Chris or an
Alex, at least they will have a more 'girly' nn to have instead if they so wished.
Using your example of
Mick /
Mike,
Michelle and
Michaela are feminine forms of
Michael so have just as much right to use it as boys. Same with
Bobbie or
Bob as a nn for
Roberta.
It can go the other way too,
Fran can easily be a nn for
Francis even though it's more common for a female or a Bren could as much be a
Brendan as a
Brenda.
I believe any one can have any nn they wish. It is after all their choice, not something that's been forced on them at birth. If I have a daughter called
Harriet who feels more like a
Harry than a
Harriet or
Hattie, I wouldn't be bothered. At least she's had a choice between
Harriet,
Hattie and
Harry.