What do all the hitmen in John Wick 2 who live like tramps do all day?

First, they aren't just tramping about. They are scouts in a massively connected operation. Some have to sit here, some walk around with carts. But they are the eyes and ears of that faction. The homeless thing is just a cover, and a pretty good one at that since most of society tends to avoid the homeless, even as far as not making eye contact. You get the bonus of discretion there as well since it's not likely witnesses will be able to identify you from general recollection.

And to some degree this has to apply to everyone in the John Wick world. How was Winston able to stop all those people in the end of chapter 2? I guess because they have a lot of connections and can sway a lot of things. So not just homeless. There must be city workers as well. People who can tell civilians you can't go down there right now or something. It has to suspend disbelief a little to get away with the preposterousness of Wick's situation.

I would be a little more interested in how John can still hear anything after the life he led. But as we can see, not everything in the John Wick world should be evaluated. It simply can never connect with reality and has to, in some way, be thought of like the Matrix. It is what it needs to be for the here and now. Otherwise what would the movies be like?

What do all the hitmen in John Wick 2 who live like tramps do all day?

There is no indication that these people actually live like tramps all the time.

We only get snapshots of tiny pieces of time here. These people could all be on assignment posing as tramps in the period that we see them.

Of course, the point of the scene is to indicate to the audience that ANYONE could be a hitman or at least be aware of the bounty on Wick and be in a position to collect. It's not intended to be a reflection of reality.