On iPhone, Android and Desktop
A widget is a web page and every version of Notion renders embeds, so the answer is yes on all of them: browser, Windows and Mac apps, iPhone and Android. Two differences are worth planning around, and both come down to size rather than support. Phone screens squeeze width, and Notion stores the height you set on a big monitor.
Width
Phone screens are narrow, so a widget built inside a wide desktop block gets squeezed into a column. Simple content survives that. A widget with four things side by side does not.
Height
Notion stores the embed height per page and the phone apps respect it, so a block sized on a large monitor can take up most of a phone screen.
The home screen question
People search for whether these appear on the phone home screen next to the weather and the clock. They do not. These live inside Notion pages. Home screen widgets come from the Notion app itself, and thePlus.so does not build those.
Open the page on a phone once after setting the size. It takes a moment and it is the only reliable way to know.
Other apps on a phone
Coda and Craft render embeds on mobile in much the same way. Google Sites can be stricter, and some WordPress themes lazy load embeds, which delays the widget rather than breaking it.
If a widget has to work on a phone across several platforms, keep it to one idea and one line of content. Anything needing two columns will be squeezed somewhere, and it is usually on the platform you tested last.
Two smaller details. Embeds do not autoplay sound on mobile, which is a browser rule rather than a Notion one. And a widget with a fixed pixel width will scroll sideways inside its block rather than shrinking, so build with percentages where you can.