I have a small wacom tablet and I use it to make maps outside of roll20, but you could do it with the drawing tool in the system itself. One problem I could see happening is that the drawing tool in roll20 is quite limited, in that you cannot use layers (in the photoshop sense), brushes, or other effects to create detailed environments. If you're just using it as an impromptu tool it should hopefully be workable though. The second concern is that tablets do take a bit of configuration to get working how you want them (at least mine did), so I recommend having whatever utility it comes with set the tablet boundaries to what you are comfortable with, and use roll 20 in fullscreen mode to avoid accidental clicking issues (F11 in most browsers). For mine I have mapped the tablet to the screen position, such that if I put the stylus on the corner of the tablet, it will make the cursor do the same (I believe this is called absolute positioning but i'm not sure). Hope you find what works for you and GL!