It's probably an exaggeration, but they've cooked pretty well for the two mystery boxes. They're giving a lot more detailed breakdown of the food this season, we're even getting to see a lot more people cooking during the challenges instead of the same 5 people + whoever's getting eliminated, and it might make it look like there are worse cooks than usual.
I don't think this batch is worse than seasons 1, 2, 6, or 7. I don't even remember season 9 (I stopped watching that about 6 or so episodes in, IIRC).
Best batch ever was probably 5 or 3, but we've yet to reach the finale for this one. The growth arc was often lost on previous seasons because of the editing and time allocation (especially in favour of judge grandstanding/drama), so maybe we will see better cooking down the line. Masterchef AU showcases the arc of going from great homecooking to turning in restaurant quality dishes wonderfully well towards the finale.