rm 125 wont kickstart but will jump

humm sure sounds like a plug but you could try adjusting the idle screw

skittle, i had the same eact problem with my cr. Although mine is a 1984 cr125 the electrical was a little rough and i replaced the CDI and the Ignition coial and i still could not kick start it and only jump it. It sucks cuz once yoyu stall it on the trails or on the track your pretty much screwed. So what i did was i cleaned EVERYTHING. I took the flywheel off, cleaned the magnet, took the stator off, brushed everything off and sanded the outside. I also took off my pulsegerator off my flywheel (dont know if you have one) and saned/cleaned that electrical terminal and cleaned of its ground. I cleaned all the grounds in the electrical system and made sure that the engine mounts were not rusted as well. baught a new plug, gapped it and after a few prayers i kicked it and it started.

Now if i were you i would start from the spark plug, if its its normal tan couour or if its really black replace it with a known good one or a new one and try that. If that doesnt work, then try cleaning off the flywheel and stator, sometimes little pieces of dirt/ debris get in there and ubstruct some of the "magnetic voltage" to the stator.

Whats the compression like? If it's way too low you'll never be able to kickstart it but it will bump start.
This kid brought a 93 RT100 into shop class and the rings were stuck in the piston and it had 0 compression you could push the kickstarter down with your pinky. It wouldn't kick over but one kid bump started it and the throttle stuck and he almost went through the shop door.

Oh and also clean your carb really well including all jets. You say it won't idle that sounds like your pilot jet is plugged.

Check the reed valves.