It depends on what you mean by impact. A market makers role is to provide market participants the instantaneous ability to buy or sell a security. They compensated a small amount for providing this service. If someone is using very poor execution techniques the price could worsen, however most large asset managers and brokers that serve them have quite advanced execution technology. The actual impact of modern market making on final execution price is typically positive relative to not having it due to competition and ultimately a tighter spread.

See Vanguard on this: http://www.cnbc.com/2014/04/25/vanguard-chief-defends-high-f...