I wouldn't be surprised if there is at least a correlation - e.g. maybe those who spend a lot of time commenting online do so because they are trying to compensate for a lack of meaningful offline interactions, relationships, etc.
That said, is it so hard to imagine that someone might have both? i.e. even if there's a correlation, it seems like a bit of a stretch to suggest that a prolific internet commenter is necessarily mentally unwell. It seems very plausible to me that someone might both have a robust real life social life while simultaneously engaging in online interactions as well, no?
I'd also posit that this might depend on the forum - e.g. someone actively engaging in learning or sharing their expertise on e.g. nutrition or car repair forum may be a bit different than the median commenter on news articles linked from The Drudge Report.