In "Julius Caesar", what does Mark Antony think of Lepidus?

This one is - unusually for Shakespeare - unequivocally and clearly expressed in the text. Antony says of Lepidus

This is a slight unmeritable man,
Meet to be sent on errands. Is it fit,
The three-fold world divided, he should stand
One of the three to share it?

Lepidus is "slight" (small, insignificant) and "unmeritable" (literally - not worthy of merit) good for sending on errands but certainly not good enough to rule over one third of the world.

Octavius disagrees:

OCTAVIUS: You may do your will,
But he's a tried and valiant soldier.

"So is my horse", Antony responds, it seems sarcastically... but then goes on to justify his argument:

He must be taught, and train'd, and bid go forth;
A barren-spirited fellow, one that feeds
On objects, arts, and imitations,
Which, out of use and staled by other men,
Begin his fashion. Do not talk of him
But as a property.

Antony's argument with Lepidus is that Lepidus, like a horse, needs to be encouraged to do everything - and who copies off other men all the time. He is simply a "property" - a "prop", a "tool" to be used - but certainly not a man.