I've successfully used Cat-5e many times for mic cables. Sure, pre-made cables with the pairs of 1-2, 3-6, 4-5, 7-8 will work, but on many radios, the signals are often not optimally paired. To have them work well, you need to pay attention to the pairs. Each pair is twisted at a different rate to minimize crosstalk. You want to make sure your mic line is paired with the mic ground, and if you have a chatty data line in the cable, it should also be paired with a ground. Looking at the standard Kenwood RJ-45 pinout, I'd wire it WOrange, Orange, WGreen, Green, WBlue, Blue, WBrown, Brown.
I've made replacement mic cables for Icom HM-98/HM-133 microphones before, and they are tricker because of the bulk needing to be stacked..... I'd suggest WOrange, Orange, WGreen, WBlue, Wbrown, Brown, Blue, Green for those.