NO you can drive the atv into mud, its sort of what there made for
BUT if you add LARGE mud tires, and or add a snorkel, or like mods, (lift kits) then YES a dealer CAN void a warranty
also, if you drive atv into water too deep and ingest water into the motor or diff's
then again they can VOID your warranty, due to owner's neglect to stay out of water that is deeper than vents on atv are made for!
so, the basic trail riding and shallow mud, or just MUD being on an atv cannot VOID a warranty
if dealer checks motor oil/diff's and find water, and its NOT due to some factory seal failing, then YES they can void your warranty due to again owners at fault for water getting into things! due to they drove into too deep a water/mud hole!
NEXT< many dealers will also turn a BLIND eye to larger tires,(NOT HUGE ONE"S) lift's and such, if there is a REAL mechanical part failure that they DON":T feel is related to the larger tires or lift
but it is at the dealers discretion here, any time you MODIFY a vehicle from the OEM's design, it can VOID the warranty many folks don't believe this, and many think, its the dealers JOB to prove there MOD caused a defect
and its NOT that way at all from my experience
as it woudl legally fall into a law suit and YOU(who ever made the mod) maybe might have to PROVE it did cause things, NOT the dealer or OEM
OEM"S MAKE things to there SPEC"s you alter the spec's
you now modified the design on how things work
larger tires, change gearing, alter handling, add wear and tear, so, they can VOID a warranty if a dealer wants to PUSH things!
and even crazier, many dealers will SELL you NEW atv;'s with larger tires, many will again turn that blind eye on repairs, but if you TAKE it to a different dealer they might NOT, if you follow me here
as I say some times, there is a GREY area in life LOL
hope this helps explain some things for you!