Would a theme in To Kill A Mockingbird be there's goodness in everyone?

You're close to the mark here, but a little clarification is necessary. I think it would be more accurate to say that, in Harper Lee's To Kill a Mockingbird, everyone is capable of goodness (and, along the same lines, of achieving dignity). That said, being capable of goodness is not quite the same as being good.
Let's first take a look at how everyone is capable of goodness. As an example, consider Mrs. Dubose, the cantankerous and mean-spirited neighbor down the street. At first, we see Mrs. Dubose as nothing more than a vile old woman. As we get to know her, we learn she's a morphine addict struggling to kick her habit. As such, an apparently mean person can be seen as brave and, in a certain way, actually good. In this way, Lee seems to be saying that all people, even the meanest, most isolated people, are capable of performing good actions. As such, Lee is encouraging us to "walk around in other people's shoes" in order to see things from diverse perspectives and try to find the goodness everyone is capable of.
There is, however, one major exception to this rule: Bob Ewell. While most characters in the novel have complex layers, Bob Ewell is essentially evil. An abusive, alcoholic father, Ewell does not care for anyone or anything except himself, and he is primarily responsible for sending Tom Robinson to an unjust trial. He has no good qualities to speak of, and seems to be entirely content existing within a small world of ignorance and cruelty. As such, it would appear that, while everyone is capable of goodness in Lee's world, some people, such as Bob Ewell, are too blinded by ignorance and hate to make use of this capability.
Now, there is an important fact we must take into account when it comes to Bob Ewell: there is a rigid class hierarchy in Maycomb, and Bob Ewell is on the wrong end of it. Indeed, his family seems to have a history of extreme, abject poverty. As such, one could argue Bob Ewell is a product of miserable poverty made worse by an unequal society that has a history of preferring "good" families (i.e., families with money) to those who have a history of poverty. That's not to say Ewell should not be held accountable for his evil actions. Rather, it's just important to note that even this detestable character is a little more complex than he first seems.
Be that as it may, Lee seems to be saying everyone is capable of goodness, but some are too blinded by ignorance and prejudice to fulfill this capability.