The threads for under-appreciated comics are a nice idea, but I'm not convinced they actually work very well in practice. The forums are mostly frequented by webcomic creators who, let's be honest, are often more interested in drawing more readers to their own comics than they are in finding something new to read. I've plugged other people's comics in threads, and usually they'll only get one or two clicks each.
I'm not sure how much sense it makes business-wise for Tapastic to plug comics that don't already have a substantial reader base, as it's a lot riskier than plugging something more popular, but there are a number of comics that I read that I would love to see get more attention, that I think are genuinely great comics that simply haven't had the right level of exposure. Something to highlight lesser-known comics would have to be very carefully curated to make it worth diverting time and energy away from plugging comics that have already proven themselves.
I do think the Tapastic site could do more to make it easier to browse comics that aren't featured on their home page. I don't pick up many new comics from there because I've already subscribed to the ones I actually like. The only front-page section worthwhile for me is New and Noteworthy, which is already a way for lesser-known comics to pick up more readers, but I assume they have to reach a particular threshold to be featured there (I know there are some comics there with 2k+ subscribers from already popular creators, but from what I recall as a subscriber, MuayThai had maybe 20-30 subscribers when it got featured there, and now has over 400).