Idiom
be a matter of opinion
1.
if something is a matter of opinion, different people have different opinions about it.
I don't think there is a perfect way to teach a child to read - it's a matter of opinion, really.
2.
if you say that something someone has just said is a matter of opinion, you mean that you do not agree.
'She's a wonderful mother.' 'That's a matter of opinion.'