The simple answer, I believe, is no. Dill would not be a more credible character than Scout based on what we've learned of his persona in Chapter 14. Througout the story, Scout seems very matter-of-fact, at least in comparison to Dill, who frequently fabricates elaborate tales about his life at home. Dill's imaginative tales make him a less-than-reliable character in one sense.
At the same time, however, Dill also demonstates in several places in the novel an empathy for others (human and non-human alike) that corrects for the lack of empathy in Jem and Scout. In Chapter 1, for example, Dill challenges Jem's claim that turtles feel no pain when a match is lit under them, and after witness the trial, Dill challenges Scout's view that Tom Robinson is "just a Negro" and therefore not worthy of being treated with respect by the prosecuting attorney.
Dill is one of my favorite characters in the novel. He seems at once childish and wise.
EDIT: The previous poster beat me by about two minutes, but I think we agree. Dill is compassionate but not necessarily credible.
I would have trouble calling Dill a credible character. Scout has come from a much more stable ad well balanced life. While she is outspoken and can be rude at times, she knows who she is and has a stable foundation which has been provided for her by Atticus and Calpurnia.
Dill is a bit of a wild story teller. His background has been one of turmoil and upheaval. He was sent to live with his grandmother in the summer because of the difficulties between his parents and their changing lifestyles.
Dill is a compassionate fellow though and he evidences this by his reaction to the ill and disrespectful manner in which they treat Tom Robinson during the trial which makes him cry. However, this does not make him more credible. Scout is more credible.