Who is Mrs. Mann?

To answer this question, take a look at chapter two. In this chapter, Oliver is sent from the main workhouse to a "branch workhouse" where he, and thirty other children, are placed under the care of an elderly, female superintendent. That superintendent is Mrs. Mann, and it is her responsibility to care for the young Oliver and the other orphans.
From the text, we see that despite her maternal role, Mrs. Mann is neither maternal nor caring towards the children she looks after. In fact, she is far more interested in the financial benefits of this role than in actually tending to the needs of children:

The elderly female was a woman of wisdom and experience; she knew what was good for children; and she had a very accurate perception of what was good for herself.

In addition, we learn that Mrs. Mann is violent towards the children and does not provide them with adequate nutrition. When anybody from the authorities arrives to inspect her establishment, however, Mrs. Mann pretends to be the perfect mother figure. The children are washed, well-dressed, and appear happy. We see this when Mr. Bumble unexpectedly arrives. Mrs. Mann acts as though her only interest is in the children's welfare when, in reality, they are all desperate to escape from her:

That I should have forgotten that the gate was bolted on the inside, on account of them dear children!

However, Oliver's experience in her care shows the very opposite. She is a cruel and financially-motivated woman who shows no love towards those she is paid to look after.