Is it a huge source of revenue?

The category never seems like it gets above 20k concurrent and that’s mostly for a couple of streamers. Right now it’s the 55th most popular category with 4.4K concurrent viewers which is two orders of magnitude less than the most popular category.

That is if you equate viewership with revenue, but most Twitch streamers I have heard talking about this is that the income from advertisements (scales with viewership) is lower than income from subscribers/donations (also scales with viewership, but more so with willingness to donate).

Right but we don't have access to conversion rate by category to compare. And it'd have to convert ludicrously well and the other categories ludicrously badly for it to get anywhere near being relevant to Twitch's revenue. Personally I'd suspect the advertising revenue for the Top 10 streaming categories (over a million concurrent viewers right now) to far outstrip the sub revenue from the Hot Tub category (4.5k viewers) all by itself.

DrDisrespect?

Doc had a multi million $ contract with Twitch, while I bet these bathtub streamers don't, so they're cheaper, and get even more money in donation from very lonely viewers.