Theresa May did not support Brexit, despite being a member of the Conservative Party. This indicates she is willing to see past the official party line, which further indicates an open mind. Now that she is stuck with Brexit, her ability to keep an open mind will inform her decisions about transitioning the UK into the future.
Looking at Theresa May's past votes, Britain's new acting Prime Minister's record on supporting human rights is not exactly great. She also generally voted against raising welfare benefits, even voting to reduce them, and against measures to prevent climate change. This austere approach to government is not going to be good for her home country in the long run.
With the current political environment, a Prime Minister who finds herself in extraordinary circumstances is more likely to be confronted with "no confidence" votes than she is to be allowed to rise to the occasion. Confronted with a wide range of issues to face, she's also more likely to handle some better than others. There's no reason I know of why she won't be a good Prime Minister, but probably won't be "great." Churchill is generally considered to have been great, but he had the war to focus on.
Theresa May will not be a great prime minister because there are too many divided factions within the United Kingdom government to allow her to carry out her full agenda. Even if she were to carry it out, there is no telling that such agenda is great. Only time will tell.