Be careful — the answer has some wrinkles to it, because there is more than one way that graphics tablets are used.
The Apple Pencil on a iPad can control a Windows or Mac computer display only if you set up the iPad display in Duplicate mode (Mirrored mode on macOS) — both the computer display and iPad have to show the same thing. If your computer display is set to a high resolution, controls may be tiny and hard to hit on the smaller iPad screen; to be productive you may have to lower the display resolution to something that lets you see and control the UI more easily on the iPad display.
If you set up the iPad display in Extend mode, the Apple Pencil can’t cross over to control the computer display, so the Lightroom Classic window would have to be on the iPad, with nothing showing on the computer display. You can get around that by opening the Lightroom Classic Secondary Display window on your computer display, set that to Loupe view, and watch a large preview on that as you move the sliders on the iPad in the Develop module. But there are serious problems with this approach: The Secondary Display window is not as high quality as in the Develop module, does not show overlays or on-image controls, and is a lot slower (I don’t think it’s GPU accelerated), lagging behind the Develop module. If you worked with masks you could only do that on the smaller iPad screen, because mask controls don’t show up on the Secondary Display window. So overall, an iPad display set to Extend mode can be a slow and painful way to work.
If you switch to a Mac, although macOS and iOS have the Sidecar feature that lets you use the iPad as an additional Mac display with no extra software needed, Sidecar in Lightroom Classic is subject to the same limitations above, working much better in Mirrored mode than Extended Desktop mode.
Actually, using Lightroom Classic can sometimes be more difficult to use with a stylus, because Lightroom Classic has many very tiny controls that are only a few pixels across. With a mouse, you can easily nudge the pointer into position over a tiny control and then drag. As you get ready to drag with a stylus, you have to hold the stylus slightly above the tablet surface (you can’t put it down); if your hand wobbles as you position the stylus tip you could miss the tiny control when you press down the stylus tip to start dragging. This can be worse with a smaller iPad screen because then the controls will be even smaller than on the computer display. (High-end Wacom tablet displays are larger than any iPad.) Again, you could reduce the iPad display resolution at the expense of working area. Apps designed for touch/stylus, such as the Lightroom app for iPad OS, have large controls designed for easy stylus/finger dragging.
A stylus tends to be many times more useful for retouching and expressive painting (using stylus pressure sensitivity) in pixel-based editors such as Photoshop, than in parametric editors such as Lightroom Classic where you mostly drag sliders. The main advantage of a stylus in Lightroom Classic is for painting masks, not for hitting its tiny slider controls.