To view all elements in the index change the print options that “sparsifies” the display of the MultiIndex.
pd.set_option('display.multi_sparse', False)
df.groupby(['A','B']).mean()
# Output:
# C
# A B
# a 1 107
# a 2 102
# a 3 115
# b 5 92
# b 8 98
# c 2 87
# c 4 104
# c 9 123