The Slim-JIM and other J-poles are dipoles fed from the high-Z end. If the transformer section is working as it should the radiation pattern is that of a dipole. What Andy says about the radiation pattern applies equally to a dipole. No ground plane is required.
For vertical use it makes a lot of sense to feed from the bottom, being more convenient than trying to bring the feeder out perpendicular to the feed point at the dipole centre. It will work fine on its side, but then it doesn’t really offer any advantage. A horizontal dipole is nicely balanced (in a mechanical sense I mean), so you’re probably better off building a good, solid dipole to use this way round. I have used my 70-cm Slim-JIM tied to PVC pipe on its side quite effectively though.
If tempted by the HB9CV you might also like to consider a Moxon: