While I'd really like a desktop tipping feature too, I think the problem is that it's not just a matter of transporting the mobile tipping functions onto desktops.
For one, there's nothing to say that the advertisers on mobile would want their ads run for desktop users. Almost all of the ads I've seen on the Tapas App are for other mobile apps, which can be easily installed. It'd be harder to get desktop users to engage in that, and if desktop needs its own ads, that's more work for the staff with an uncertain benefit.
There's also the problem that if you buy coins on the App, it goes through Apple/Google which does the processing for them. But what would they do on the desktop version? They'd either have to implement a completely new platform, or use an existing software into Tapas (i.e. Paypal), in order to convert that to coins, which then can be donated to the author.
But in this case, wouldn't it be easier for the user to just donate to like, the artist's own paypal or patreon? :"D
So sadly, I don't think we'll get a desktop tipping feature any time soon. I do wish Tapas had more features encouraging either tipping/some free method for supporting creators, or promoting patreon or paypal donations, but I guess only time will tell. :"DD