It's perfectly reasonable to forbid access to Brazil for some service that does not follow a Brazilian law. This is bad because of the specific terms of this law, not because of some broad issue.

I don't know if they have servers in brazil, but if they don't then it seems quite unreasonable to say that they are "providing a service in Brazil". If you run a bookstore in the US and a German comes and buys Nazi material, are you running a store in Germany? If Brazilians are effectively leaving their country to go get stuff from US servers then it's up to the Brazilian government to make a law to stop them, if it doesn't like that. Saying that connecting to the internet is "providing service" to every country in the world is a way to simply break the entire internet.

No, but if she buys via mail order and the package gets confiscated at the border, don't whine about losing business.

The most important part however is that of course you don't have to give a damn about what German law thinks as long as you aren't in a position in which German jurisdiction can be enforced. Likewise, even if you are a German citizen living in Germany some German laws and regulations may not apply if you are decidedly not offering your services/apps to a German audience -- though of course that's a much less safe position. Either way, it's not as simple as "it exists on the Internet, therefore it falls under German jurisdiction" although the reasoning is quite similar to that in Brazil.

The point is moot, anyway. Brazil can't enforce their laws against a US company that doesn't have a presence in Brazil, but it can ban them from Brazil -- as apparently a Brazilian court is allowed to force Internet access providers to ban specific IPs. Whether courts should be allowed to do that is a legitimate question but right now in Brazil they apparently are, so everything is fine.

This isn't an action against WhatsApp. This is an action against Brazilian WhatsApp users. It's basically enforcing a sanction against WhatsApp by preventing Brazilians from accessing the service (which they can't get at otherwise). This is more like a German court forcing an IP ban (in Germany) against a Nazi website hosted outside of Germany -- which is a thing.