Consider ratings as sentiment labels?
That would depend on the exact goal of the task and the specifics of the dataset, but in general I would say that it's always better to use the information specifically provided with the data if it's relevant for the task. In this case the rating for the product is indeed very likely to reflect the sentiment of the text, so I would go with it. Notice that you could also do both and compare the cases where the predicted sentiment differs from the one derived from the rating.

Given that the ratings are provided as 1-5 scores I would also consider the option of treating the task as a regression problem, instead of the standard binary classification setting.