Since Xamarin.Forms was introduced many has asked for a visual designer for building user interface like the one we have for building WPF or UWP apps. But I don't think it is many that uses it like a designer, I think it is mostly used like a previewer of the XAML that you are writing. At least I use it that way.
Xamarin has tried with Xamarin.Forms previewer, but I have never got that work in real scenarios. But it will be great if they could get it work for real. Xamarin also has the Xamarin Live Player that makes it possible to debug iOS and Android apps on a device via WiFi. You don't even need a Mac to debug iOS apps. The only thing you need to do is to download the Xamarin Live Player app and pair it with Visual Studio (works on both PC and Mac). With Live Player you can also live view XAML views, it makes it possible to make changes and see the result on a device without redeploy it. But I have had some problems with it, it can be complicated to pair Visual Studio with the app, especially if you're on an enterprise network. I have also facing issues with more complex scenarios, for example if I have creating my own base views. But Live Player is still in preview, so I am looking forward to a stable product.
My favorite tool right now is LiveXAML (https://livexaml.com) that makes it possible to edit XAML when you're debugging. You just start the app and navigate to the view you want to edit. When you save the file, you will see the changes live in the app. It works both on Visual Studio for Mac and PC and on both on simulator/emulator and on real devices. It cost $240 or $24 per month. But it's nothing seen to the time you will save when you don't have to redeploy your app to see small UI changes.
Gorilla Player is a similar tool to LiveXAML that are free and could be worth taking a look at. But last time I used it I think it was more complicated to get it to work if you compare to LiveXAML, but they have probably improved it since I used it.