The Soft Body object and its Collision mesh(es) are most often separate objects, so the mane and body should be also. You can do Self-collision with SB objects but it’s really finicky and can produce odd results under some circumstances. I usually avoid it if I can. I’d extrude the neck to join with the head, and link all this to the main armature for the overall body motion, basically the lion minus the mane.
I’d then model the mane to fit snugly around the neck, like a fat sleeve – there are ways to get an exact match between the outer neck surface and the inner mane surface – and also link it to the main armature using similar weight painting. The intent here is to get the mane as a separate object but for the most part moving with the body as if it were part of the body.
One that’s done, you can apply the Soft Body physics to the mane, using a weight-painted vertex group (likely one which contains all or nearly all the mane mesh vertices) to control how much the Soft-Body physics affects the mane. This is what the “Goal” vertex group is used for in the SB sim.
For example, at the top end of the mane where it meets the neck/back of the head, you probably want the mane to act only under the Armature control – here you would paint the Goal vertex group with a weight of 1.0 (solid red). Soft Body physics would(should) have no influence on that part of the mesh. Down at the lower front where the chest and legs meet the mane, the collision between body and mane should govern the motion, so you would paint considerably less Goal weight, allowing the SB physics to work more strongly there.
In other places around the mane you may want a little “jiggle” from SB motion, so an intermediate weight can be given to the Goal, allowing both Armature & SB to have some influence.
It’s a bit of a balancing act, getting the Goal weighted so everything moves as you want it to, but the results are worth it imo. While it would be possible to rig the mane and key its motion fully via an armature, I think it might tend to look too stiff and less natural, since with the SB physics it can have a little “independent” motion governed by the body’s action.
In terms of modeling the mane and body to work well with the SB sim, my rule of thumb is that a denser mesh will provide better collision detection (a good thing) but too much density can really slow down the collision calcualtions (not a good thing). So try to add loops and increase mesh density only where it’s actually needed, in the areas where collision is more important. It’s also a good idea to use a collision proxy, essentially a duplicate of the main body mesh that moves just the same, but from which have been removed all faces that will have no SB interaction. This can really speed up the physics sim calculations. The proxy can be set to not render, or be placed on a non-rendering layer as long as it and the SB object, the mane, share that layer.