The original text reads:
So well she acted all and every part
By turns--with that vivacious versatility,
Which many people take for want of heart.
They err--'tis merely what is call'd mobility,
A thing of temperament and not of art,
Though seeming so, from its supposed facility;
And false--though true; for surely they're sincerest
Who are strongly acted on by what is nearest.
This makes your actors, artists, and romancers,
Heroes sometimes, though seldom--sages never;
But speakers, bards, diplomatists, and dancers,
Little that's great, but much of what is clever....
( Don Juan, Canto XVI, Stanzas 97-98 in Byron, The Poetical Works, p. 854)
Ruskin, like Byron, appears to be distinguishing between actual ability and apparent facility, observing in their discrepancy the gap between art and its clever imitation, although he undercuts his point by ascribing mere 'cleverness' to artists while Byron had attributed it to 'speakers, bards, diplomatists, and dancers'. Perhaps Ruskin is misquoting deliberately, to stress his belief, stated a few lines later, that 'facility is in art inconsistent with invention'.
Given Ruskin 's depreciation of the old masters at that time, it is unsurprising that he might agree with Byron that such 'mobility' is indicative of shallowness of thought and lack of originality; in Byron's own notes to this poem, he identifies 'mobility' as an attribute associated with the French rather than with the English character: 'it is expressive of a quality which rather belongs to other climates, though it is sometimes seen to a great extent in our own' ( Byron, The Poetical Works, p. 918.)