How bad is a Dragon type Hidden Power for Suicune?

Anyways it is nearly always being outclassed by either Omastar/Golem, Rhydon or Vaporeon.

Steel and fairy resist dragon.

Your analysis is fine.

Real question is when would you ever use this?
I can't think of a single time where I would use "Neutral" Dragon Move + Water Charge Move.

If it Resists Water: I won't use a Pokemon with WC move
If it Weak to Water: I want a pokemon with Water Fast Move.
If it is Neutral to Water: I will use something more favorable.

If you want to use Suicine just to use it:
HP: Rock vs Flying+Fire types would give you 2*SE Quick & SE+Stab Charge.
I'd power the 13 attack one up over the 15 attack one based on HP typing.

Semi-Useful HP:
HP: Rock vs Charizard/Ho-Ho/Moltress
HP: Fighting vs Tyrantir

Medium Usefulness situations
HP: Water vs anything waterish just for VARIETY or if you started the game after gen2/gym change and only have 1-2 Vaperons.

If you want some super rare niches you could add
HP Electric: anytime a Gary is in Gym followed by a Rhydon/Fire/Golem where you can spam quick and save charge

Grand scheme of things though if you want to power one up for fun I would limit it to 3 options
Fighting, Rock, Water

If we could use legendary defenders, then dragon Hidden Power might be one of the best options.

I'm personally not going to consider powering up Suicune until it hopefully gets a water fast move like Waterfall, in which case the type of Hidden Power doesn't matter. I'll just keep doing the raids to stockpile more TMs for as long as people are gathering--rolling my eyes as they synchronize their lucky eggs to hit two Suicunes.

Water Hidden Power is really the only one worth powering up. The STAB means that one with a 10 ATK will outperform a perfect one with non-STAB. The only hope is for a move reshuffle and a buff.