Yes, Mixed mewtwo kind of a beast, however Dragonite cannot be completely ruled out due to the fact he is just so powerful in Generation 2 and Uber pokemon might even have trouble killing this beast due to his movepool and exceptional stats.
Well, if we focus Dragonite's damage primarily on Uber Pokemon, Dragonite's strongest moves are Thunder, Fire Blast and Dynamicpunch (not going to count Hyper Beam and Blizzard). Dynamicpunch is out of the question with Ubers being Psychic and/or Flying, so-
Dragonite's Thunder vs:
Mew: 22-25%
Mewtwo: 22-25%
Ho-oh: 31-36%
Lugia: 31-36%
No need to mention Celebi. Ho-oh and Lugia get Recover and there's always that 30% chance to miss. Dragonite's simply not strong enough to scare them off with Thunder.
Dragonite does do a respectable 43%-51% to Celebi with Fire Blast though. I guess that's something.
On the physical side (omitting Dynamicpunch) you have Wing Attack, Double-Edge, Body Slam, or a Hidden Power of your choice. Body Slam/Wing Attack isn't breaking past the 25% mark on anyone and Mewtwo/Ho-oh with the lowest Defense of any Uber are taking 27-32%, and once again they have access to Recover. That leaves you with HP Ghost for doing a decent chunk to Mew/Mewtwo or HP Rock to decimate Ho-oh. That alone would make it AT LEAST a decent Ho-oh counter, save for the face that it has to switch in to 50% burn chance each time.
Edit: As far as power goes, Dragonite doesn't have the moves to break things like Ubers can. I suppose it could have some sort of defensive appeal over Lugia for getting Reflect/T-Wave/Haze for vs Cursers.
Edit 2: What's the opinion on Heracross? Since Psychic is just about non-existant, only Lugia, Ho-oh, an Drill Peck Zapdos look like they'd stand in its way, and for Ho-oh it could hit it with HP Rock.