Hypotonia is the condition of having low muscle tone so that the resting muscle is flabby or floppy. To understand the idea of muscle tone please see the health page "Muscle Tone and Spasticity."
http://www.medhelp.org/health_pages/Multiple%20Sclerosis/Muscle-Tone-And-Spasticity/show/159?cid=36
Hypotonia is very uncommon in MS. During a simple search I could not find it mentioned in association with MS except in the side effects of some of the muscle relaxants - especially Baclofen.
The mechanism for causing the diffuse hypotonia of CP is diffuse brain damage and not the sporadic, damage of MS. In MS the change in muscle tone is almost always lesions in the spinal cord leading to hyperreflexia, hypertonicity and spasticity. In MS the early fatigue we feel is a function of demyelinated motor nerves that lead to early exhaustion.
Rather than trying to ramp up and use a muscle from floppy to active - which requires a great deal of effort, we suffer from another problem altogether, though the outcome is the same. We tell a muscle to contract, but it doesn't get the full nerve signal to do it. This is specific muscle weakness. Also, if spasticity is part of the problem when wh try to work one set of muscles it's action may be countered by the pull of the opposing spastic set of muscles.
Physical therapy has different goals in MS. We work to maintain what strength we can, and increase what strength we can. We also learn adaptive maneuvers to compensate for the muscles that are not behaving. Another important task of PT is to learn to stretch out spastic muscles properly and frequently. In MS we tend to be stiff and tired and become less flexible.
They have identified many reasons why we become fatigued in MS, but hypotonia is not one of them. Her is a short discussion of these causes of fatigue:
Fatigue in People with MS
http://www.medhelp.org/health_pages/Multiple%20Sclerosis/Fatigue-in-People-with-MS/show/63?cid=36
Hope this helps
Quix