Donna this year hasn't lost to any player that is ranked worse than her. Actually it happen once, in Kuala Lumpur, when she lost to Mattek-Sands (then WTA 197th) but Mattek-Sands is now already 55th in the rankings which shows no shame in that loss too. In all other matches Donna has beaten all her opponents who are worse ranked than her or even better ranked than her in that moment (Hlavackova, Putintseva, now Cristea, Radwanska, Rybarikova). So, so far she has 5 wins over better ranked players but 1 loss (and that player improved significantly in ranks since that match and in few months became also better ranked than Donna) against lower ranked player. Does that actually suggest that Donna is over or under ranked?
Furthermore, not to mentioning she is STILL just 16 and has restrictions to play full calendar as others in top 100 don't have! Well done Donna!