Is frost DK still good in Midnight?

for questing, delves and normal dungeons it's fine, honestly any spec is. the gap between specs only shows up in high keys and mythic raiding, and even there it moves every tuning pass. if you like playing it, don't reroll because a tier list dropped it a rank. if you're pushing keys with a group that cares, look at what's being run in keys this week, not a list written at launch.