In order to understand what this painting would be a good or bad source for, we must first understand when and where it was painted. This painting was done by Jean L. G. Ferris, an American painter who was born in 1863 and died in 1930. The painting was done sometime in the late 1800s or early 1900s.
Once we know this, we can know some things about what questions this painting could help us to answer. First of all, we know that it cannot help us answer any questions about the actual conditions or ways of life in 1621. Ferris was not an expert in the history of that time period. For example, he has the Native Americans wearing headdresses that were not worn by Indians in that region. He has the Pilgrims wearing black clothing when they really did not typically dress that way. Since Ferris is not an expert and not a firsthand witness of what happened in 1621, he cannot help us answer questions about how the Pilgrims lived, how the Native Americans lived, or what the first Thanksgiving was really like.
What this picture can help us know is the sort of attitudes that Americans had about the first Thanksgiving in the era when Ferris painted. If we find out how long reproductions of this painting were popular, we can understand about the times in which it was a best seller. The painting bears witness to American attitudes at the time when it was painted and sold, not about the time that is depicted on the canvas.