What an excellent question to consider! Of course, there are two ways of looking at this. On the one hand, you could argue that the way in which Mrs. Mallard so quickly shifts from grief to joy about the state of freedom in which she finds herself is rather callous. She thinks more about herself than about her husband, which could be used as evidence to indicate that she is callous. We aren't actually told anything about their relationship together and if Mr. Mallard mistreated her, but perhaps we can infer from the silence devoted to this topic that theirs was a normal marriage with Mr. Mallard trying to do his best as a husband. If this is true, then we can argue that Mrs. Mallard is rather callous. Any love that she does feel towards her husband is replaced by this sudden epiphany:
What could love, the unsolved mystery, count for in face of this possession of self-assertion which she suddenly recognised as the strongest impulse of her being?
Certainly, if you look at this quote in one light, Mrs. Mallard appears to be rather self-absorbed and selfish, focusing on her own feelings and life rather than considering her husband and his passing.
However, if we consider that this story isn't about Mr. Mallard and Mrs. Mallard, but is about the state of society at the time of writing and the kind of patriarchal dominance that men were given, this helps us to see things differently. Consider the following quote that comments upon the state of marriage and the relations between the genders at this time:
There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow creature.
This, then, to Mrs. Mallard is what she has escaped thanks to the death of her husband. His passing away has given her freedom from having her will "bent" and given her the opportunity to live for herself. If we understand this presentation of marriage, then the response of Mrs. Mallard looks a lot less callous and much more understandable.