From what I can tell, there is no link to the forum on the Tapas app. Not to mention that I can't even access the forum from my phone: every time I try to log in, the app opens instead and won't let me do anything at all. So far the only phone browser that let me log in was Firefox Focus, and even then, had to mess with it quite a bit to be able to log in and, since the browser deletes all cookies once you close it, I would have to log in every single time I opened it. Not very feasible at all.
On the matter of reading mobile-framed comics on desktop: I... don't find it to be much of an issue at all?
Granted, most of the comics I follow are in page format, but so far the ones I've seen that are made for mobile tend to have images uploaded at maximum size anyway, so it's not like they're blurry on desktop or whatever. Yes, it's a bit annoying because you have to scroll A LOT to get through a single update, but as for everything else, I never found it to be that different from regular comics in page format 