Maybe not really bad but certainly not good. The discussion for this is about defaults not specific search terms. When you have something more specific, yes, Google will decide what to show but there are many, many cases where the default text (as determined by the meta description) will be displayed.
The reason it is important is actually not searchability, it is clickability. As much as a website operator can control it, they should strive to make their links and link descriptions clickable in search results. Here is some info: Meta Description | Moz
Look, I hate the SEO game, I really do, I fought with it at Spoke for years … it makes me upset. But we have to play by the rules and one of the first rules is decent meta descriptions, good titles, and h1 tags.
I’m not sure what would be good for meta description for all of the pages in a Discourse forum but I’d like to customize at least the index page, and maybe the categories page.
Update: looking at that link I sent in a little more detail, I can be okay to have no meta-description for long tail. However, the home/front page is generally not considered long-tail. Every other post/topic in the forum would be.