I think you are sort-of asking two questions. pico8 is a great way to start learning how to make games. I advise you to start with the built-in tutorials to see how to draw things, how the draw and update loops work, how to make a simple platformer, etc.
Isometric or 2.5D capabilities? Lots of folks, including myself, have made varying degrees of pseudo-3D games with different strategies. pico8 is "built" for 2D games, so I would advise getting comfortable there first. I've posted some of my favorite psuedo-3D games below (including my own). Some folks have written full-blown 3D games (https://www.lexaloffle.com/bbs/?pid=32480#p @electricgryphon) but these are pretty advanced - definitely above my level of comfort. I recommend checking out some of these below to see what they look like to get some ideas.
Marballs (isometric view, 3d internal gameplay): https://www.lexaloffle.com/bbs/?pid=12502#p
Hungry Harry 3D (fixed camera 3d, my own game) https://www.lexaloffle.com/bbs/?pid=59509#p
PicoKart (Mario Kart, mode7-like rotating map) https://www.lexaloffle.com/bbs/?pid=17739#p
Lands of Yocta (voxels, first person view) https://www.lexaloffle.com/bbs/?pid=41198#p
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