I agree with Andy that it doesn't matter much for the butterfly -- which only lives a few weeks at best anyway.
However, it matters very much indeed for the species and other attractive ones. A market for mounted butterflies gives a very strong incentive for people to go out and catch them.
If they do this in a sustainable way, fine -- and in fact potentially very good, because it could be a rare source of income for cash-poor people in the tropics, and a way of making natural habitat pay for itself and so survive.
However, there is a very high risk that uncontrolled collection will have severe impacts on all attractive species. Any income people might get will then be very short lived, as may the habitat.
So I would say that it is a big mistake to buy mounted butterflies, unless (one of):
- You can be certain they are captive-bred.
- You can be certain that the "harvest" is sustainable -- this could only be through a certification scheme (don't believe any sales pitch to the contrary!). A proper, independently verifiable certificate for each specimen would be a minimum. I don't know of such a scheme for insects (but there may be one).
- You are certain the specimens are historic (collected some decades ago) though even here there is still a risk that their sale will stimulate demand for new capture.
Arguably, even having an ethically acceptable one on your wall could stimulate your admiring friends to go and buy something more dodgy.