Accuracy Testing & Training
You haven't actually used accuracy_score, and the default scorer for sklearn regressors is R^2:
https://scikit-learn.org/stable/modules/generated/sklearn.neural_network.MLPRegressor.html#sklearn.neural_network.MLPRegressor.score

A negative value for R^2 generally means a very bad fit:
https://stats.stackexchange.com/q/12900/232706