Is my Slowking worthless?

https://pokemongo.gamepress.gg/pokemon/199
It is an A-rated offensive moveset.

...Not really sure what you'd use it to attack though.

I think there is a superfluous word in your question. Should just be:
"Is Slowking worthless?"

good for kick ass dragonite, that all...

It's a very cool Pokemon like Slowbro, but probably better for defense w/ C/P or B. Yours was also evolved at too high a level so prestiging is out of the question.

I was going to ask the same question, with my new Slowking having Confusion/Psychic but only 1645 CP. But got to thinking, it might be cheaper with dust just evolving another Espeon for another good psychic pokemon.

C/B is better, a prestiger against Dragonite.