When analyzing characters for personality traits, we are identifying what they are like as people. We can easily do this by looking at the things they say and do as characters.
Two things we learn about Mibs in the very first chapter of Ingrid Law's young reader's novel Savvy is that she has a way of being able to view the world in a very positive and humorous way. Her optimism can be seen in her anticipation of what her savvy will be. Never does she allow herself to think it will be something dreadful. Instead, she allows herself to think it will be something powerful and even useful. We especially see her optimism when she says to her brother, "My savvy is going to be a good one ... I just know it." Since she is consistently optimistic, we know optimism is a personality trait and not just a temporary perspective.
It is also her wit that allows her to see the world positively. For example, when she and her family had to move away from the coast because her brother, named Fish, developed the savvy of being able to produce hurricanes, though she misses the beach severely, she is able to see the situation humorously as evidenced by the fact that she, along with her family, call the area in which they now live Kansaska, Nebransas, and sometimes Kansaska-Nebransas. These names stem from the fact that they settled in between Nebraska and Kansas, far away from other people as possible. Humor is also seen when, after being teased by vicious girls at her school for starting to be homeschooled, she hopes her savvy will allow her to "turn nasty girls into slimy green frogs or to glue their mouths shut."
Hence, being optimistic and witty are two personality traits two personality traits Mibs has.