Oscar Wilde is not a moralist. As an aesthete and as an artist, his goal is not to create good or bad characters, but to develop a character with qualities to which the average reader can relate. Making a character "good" or "bad" would be, as Wilde would say "actual fiction".
This being said, the character of Mrs. Erlynne is not meant to be condoned nor condemned. She is clearly an amoral woman meaning that, whether her actions are seen as good or evil, the opinions of the people will make no effect on her: as long as she has a way to get what she wants, it makes no difference to justify or excuse her actions.
What is arguable, however, is that Mrs. Erlynne does have a maternal instinct that is still very much alive. Having this instinct accentuated does not make a woman good or bad either; it simply means that the sense of responsibility towards the needs and want of her once-abandoned daughter is still very much alive.
It is also safe to admit that Mrs. Erlynne's dare to risk her own reputation in order to save that of Lady Windermere is an act of both care and valor. Yet, it is what any other woman with a similar sense of instinct would have done, regardless; even if for another young woman who may have resembled her daughter.
Therefore, it is safe to conclude that Mrs. Erlynne's character is meant to represent any average woman who feels the need to redeem herself and recapture the lost opportunities of motherhood that she had lost. Since she never openly confesses who she is to Lady Windermere, the reader can grant her even more "points" for courage and sacrifice. For these reasons, Mrs. Erlynne remains a representative of the modern-day woman of her time; fearless, independent, and yet still holding dear the role that nature had once bestowed upon her.