Many Marths aren't used to fighting a character that outranges him. F-tilt is a great tool in neutral. In general, I would say the key to fighting Marth is to punish his laggy stuff, but definitely respect his fast moves like d-tilt and f-air. The key is to punish him harder than he punishes you, which DK has the tools for.
Another tip is to refresh ledge invincibility while edgeguarding at the right time so that he has to recover on stage. If you can pull that off you can get a big ledgehop > d-air > whatever you want.
Crouch cancel down smash can land you a simple KO but can be too telegraphed sometimes. If he gets to high percent, grounded cargo uthrow to full jump double jump u-air is a reliable kill option, especially if you can jump up to a platform first.
I would say that pokemon stadium 2 is DK's best neutral starter against Marth. He can abuse the platforms and edgeguard you pretty easily but you net very early kills off the top using the platforms.