Usually after you go through a huge mud hole, things get sticky. Your clutch cable should be very easy to pull in, and there should be very little resistance. All you should feel is the spring in the clutch and NO drag on the cable.
If there's drag you should lube it, these cables will eventually break if there is drag. The cables on MX bikes are sealed very well, but after a few years sometimes the lube drys up and gets sticky. Lube is about 10$ a bottle in an aerosol spray can with a straw. The best way to do it is to remove the cable completely and hold it up, and spray in the upper section and work the cable so it runs all the way through.
If you don't want to take the cable off of the bike, you can take the cable out of the lever and try your best to push lube through the end using the spray straw.
A well lubed cable usually never breaks :D
BTW, these bikes have rubber seals for the cable on the both ends, it's pretty rare to get dirt in there, usually it's just dried up lube that turns to sludge