No, I'm pretty sure "char jumpfactor[4];" is actually to set the maximum amount of (text) characters that can be put in as a setting. Though knowing SRB2 enough, I'm guessing that value would actually be 3, not 4.
So basically, it means no value higher than, what, 999 can be used. (but then of course, the game tries to prevent you from using jumpheight values above 100). Turning off jumping with it obviously should be just setting "jumpfactor"/"jumpheight" to 0 anyway. =P
...besides which, I'm guessing you found that in r_things.h, right? That's not where Sonic's S_SKIN is found at all; that part of the source is just where the game defines all the possible parameters for a player skin. Most of them of course are changable via S_SKIN, but some might go by different names (e.g. "jumpfactor", "starttranscolor", which are changed by "jumpheight" and "startcolor" in S_SKIN), and some not changeable at all! (e.g. "super", ...and I think that's actually the only one, lol)