This is the 17097th most frequent Russian word.
средненькая
"Средненькая" means "average" or "mediocre" (informal/diminutive).
Here, 'средненькая' describes the quality of the painting as average, not outstanding.
Эта картина выглядит средненькой.
This painting looks mediocre.
In this case, 'средненькие' denotes the average performance levels in academic context.
Ученица показала средненькие результаты на тесте.
The student showed mediocre results on the test.
'Средненькой' characterizes the book in terms of its unspectacular content.