Do contradictions make for good character traits?

Not exactly, but it has been used to make at least decent flaws. Watt i'm saying it, lots of people tend to be ironic and don't always do things like you predicted. You'd assume a character does one thing but end up doing something else instead. It just tends to happen.

take for example someone who's a peacemaker yet seeks/ starts fights. It could either be by accident or the person is a narcissist wanting to pat themselves on the back for what they did, even if it made things worse for a lot of people.

I actually find this interesting as it could be used a sort of character arc where a character learns that using physical means to solve problems doesn't always work it. Some circumstances might need it but most don't. The character in question tries to take the peacemaker path and try to convince the obstacle a more peaceful method, only for the words to be heard as something else and the fur begins to fly immediately.

so what exactly is your thoughts on this sort of thing?

Generally, yes, because duality is part of the human condition, but with a concept that vague, it depends entirely on execution.

Something that's completely unrelated one half is completely white while the other half is completely black is not that good, though if it's more balanced and branched out, then you have a 3 dimensional person. Like someone who gives away their stuff and privileges to other people in order to make them happy but also wants attention from them as well as small gifts from time to time so they know that people appreciate them is more logical and realistic that a caricature of a tsundere who says that they happen to have extra stuff left, and you could have it if you want, and then gives it to you before stealing it back.