How is this going to have a positive impact on their economy, Bitcoin is just a payment system and just like other payment system that's been recognize by Brazilian government, Bitcoin just became one of them. The Brazil government didn't say they'll be investing in Bitcoin, If that's what they'll do then their involvement will have a positive impact as the country financial will increase when Bitcoin does go higher.
Bitcoin has grown so popular that it's unreasonable for any government to not make it a payment currency in their country then invest in them. You bring more awareness to your citizens when you accept Bitcoin and then your citizens investing in it can help them financially.
Bitcoin is not about investing only, it is a direct alternative to a classical bank system. So it gives an opportunity of using bank like solutions directly without permission or trust a third party, it returns control over own money. Competition makes system better, so bank system will have to make their system better not to lost their customers. So economy of Brazil will get a positive impact even without investing in Bitcoin.