I was planning on making a modern rogue with Phaser but looking at the jam’s page I found this:

Resources:
libtcod (C++), rot.js (JS), jsrl (JS/TS) and T-Engine (Lua) are all dedicated roguelike libraries/engines which can be very useful for completing a game within the week.

Those are for console-based classic games.

So my question is: Are graphics allowed? Should I use png sprites for a fancy roguelike?

All kind of visuals are allowed: characters, tiles, 3D, anything. Even no graphics.

Note that some of them, including JSRL, now support simple graphics.