How do I make sans serif superscript or subscript text in matplotlib?

Use \mathregular to use the font used for regular text outside of mathtext:

$\mathregular{N_i}$

Take a look here for more information.

You can do it by customizing rcParams. If you have multiple elements to customize, you can store them as a dict and the update the `rcParams':

params = {'mathtext.default': 'regular' }          
plt.rcParams.update(params)

If you want to do a single modification, you can simply type:

plt.rcParams.update({'mathtext.default':  'regular' })

In this respect, a trivial example would be as follows:

import numpy as np
from matplotlib import pyplot as plt

x = np.linspace(1, 10, 40)
y = x**2

fig = plt.figure()
ax = fig.add_subplot(111)
params = {'mathtext.default': 'regular' }          
plt.rcParams.update(params)
ax.set_xlabel('$x_{my text}$')
ax.set_ylabel('$y_i$')
ax.plot(x, y)
ax.grid()
plt.show()

You can find more information on RcParams in the matplotlib documentation.