One of my ex girlfriends went to do maths at Cambridge. Allegedly one of the frightfully clever lecturers was working through an equation one day and turned round and asked suddenly 'what is 6 x 7?'
'49!' came the answer. 'No, it's 42!' said another student.
'Now gentlemen', came the reply, 'it can hardly be both'.
Whilst I've since heard this story elsewhere, it's certainly the case that there can be a disconnect between mathematical ability and the ability to do mental arithmetic.
The PPL does require a degree of ability in both. The exams shouldn't be too much of a problem as the questions that involve maths are sparse enough that it would be possible to fail them and still pass. In practice though, flight planning is an area where you don't want to make too many mistakes. It's fine to use a calculator in real life, but if you are having trouble working out what numbers to plug into calculations relating to range, weight and balance etc... then this is something you'll need to work on.