You can get axes with g.ax_joint and from here your can set you axis attributes.
Solution 1: tick_params()
plt.figure(figsize=(10, 8))
g = sns.JointGrid(x="u", y="t", data=tdata)
g.plot(sns.scatterplot, sns.histplot)
g.ax_joint.tick_params(axis="x", rotation=90)
plt.show()
Solution 2: set_xticklabels()
plt.figure(figsize=(10, 8))
g = sns.JointGrid(x="u", y="