It’s proprietary.
This means, among other things, you cannot check its source code. This, in turn, means that it can1 contain malware that can, among other things, log everything you do online and a lot of things you do offline, and sell it to some third-party data collector to be used for targeted advertising, if not for more nefarious things…
Using a proprietary web browser is extremely foolish and careless.
The most common reason distros include proprietary browsers in their default installs is money. That said, I don’t have any inside information into why Manjaro decided to install Vivaldi. It may be just a careless decision by one of the community edition devs, or it may be something more nefarious, or I could be completely wrong… I don’t know and, since I do not use Manjaro any more, I don’t particularly care.
1and therefore it probably does, because why wouldn’t it? It is simply too profitable to miss out on it.