Report cards measure performance at a moment in time—not potential, not intelligence, not worth as a person.
A C grade in maths might mean: your child struggles with fractions right now, and needs practice. It doesn't mean they're bad at maths forever or not smart.
Comments like "doesn't focus in class" might mean: they daydream sometimes (normal), or struggle with attention (worth exploring), or find the work too easy and zone out (different issue).
Context is everything. A B in a hard subject from a tough teacher might reflect higher achievement than an A from an easy teacher.