In cases where you have multiple branching statements it’s best to create a function that accepts a row and then apply it along the axis=1. This is usually much faster then iteration through rows.
def func(row):
if row['mobile'] == 'mobile':
return 'mobile'
elif row['tablet'] =='tablet':
return 'tablet'
else:
return 'other'
df['combo'] = df.apply(func, axis=1)