Mary's behaviour towards Maria in London certainly isn't kind, but I really think it is a stretch to describe her as cruel. It all rather depends on how broadly we define cruelty, and on individual perspective.

Imagine a horizontal line as an axis. At the far left is extreme kindness and tenderness of people's feelings. Towards the right we have indifference to the feelings of others, unkindness, varying degrees of cruelty, and outright sadism at the far right.

Fanny belongs at the far left of this line, so from her perspective a great many human actions appear faulty to some degree. Her judgements of Mary are also somewhat biased by jealousy because of Edmund's feelings for Mary.

I would put Mary to the right of centre, but not by much. Mary sees things from a different perspective, and has a more robust attitude to people's feelings (including her own). She is operating in a world where the foibles of people's behaviour to each other are just grist to the conversational mill. This could be described as callous, but it differs only in degree from Mr Bennet's view: "For what do we live, but to make sport for our neighbours, and laugh at them in our turn?"

Mary thinks that "very few young ladies have any affections worth caring for".(36) Maria has suggested that she does not by the the way she has behaved throughout towards Mr Rushworth, and I think that she is to a great extent responsible for the state of her own emotions. Mary sees her as a legitimate target for amusement, in much the same way as Elizabeth sees Miss Bingley as a legitimate target for amusement when she hears that Darcy is supposedly to marry his cousin Anne: This information made Elizabeth smile, as she thought of poor Miss Bingley. Mary is actively engaged whilst Elizabeth is not, but the amusement at another's expense is not greatly different in kind IMO, and I have never thought of Elizabeth as cruel.

When Mary seems to enjoy telling Fanny how Maria lost her composure she thinks that hearing this will give pleasure to Fanny, just as she thought Fanny would like to know what a sensation news about her & Henry has caused in London.

*He [Henry] will see the Rushworths, which own I am not sorry for—having a little curiosity”,(43). Is this not wilfully or knowingly causing pain or distress to Maria and therefore cruel?*
Well it certainly isn't kind to Maria. Mary is stirring the situation and is being a bit malicious here, but I think it is a stretch to call it cruelty. If Mary wanted to avoid causing distress she would have to ban Henry from every social gathering where any of her friends who have broken their hearts over him were likely to be present!