Or just change ascending = [False] to ascending = [True]:

import pandas as pd
import matplotlib.pyplot as plt

DATA = {
    'x': ['val_1', 'val_2', 'val_3', 'val_4', 'val_5'],
    'y': [1, 2, 3, 2.5, 0.5]
}

COLUMNS = tuple(DATA.keys())


df = pd.DataFrame(DATA, columns=COLUMNS)
df.sort_values(['y'], ascending=[True]).plot(x=COLUMNS[0], y=COLUMNS[1], kind="barh", legend=False)

plt.show()