Is there a such thing as a "bad" dragonite?

You will either get one that's a good defender or one that is a a good attacker. All the moveset combinations are good for one of the two.
DB+any charge is great attacker
SW+any charge is great defender

No need to worry evolving a dratini.

Snorlax and Dragonite (especially with high IV) are strong despite their moveset

I would say for attacking
DB+DC is A+
DB+DP is A
DB+HB is A-

For defending
SW+DP is A
DB+DP is B
SW+DC is B
SW+HB is B-

So the 2 you want to avoid are SW+DC/HB
but they are still ok

Lol, I was in the same situation as you until yesterday. I have been walking my 93% Dratini and was extremely afraid to get Steel Wing (as I like attacking more than defending).

Anyway, it ended up coming with Steel Wing/Dragon Pulse, which is the best moveset for defending. I posted a topic here about my desilucion but I ended up being happy after the comments I got from this moveset.

I agree with Impala. As for all the comments I read about Dragonite (believe me I read A LOT lol) if you are getting Dragon Breath, all the charge moves will do extremely well for attacking. And if you are getting Steel Wing, Dragon Pulse will do extremely well for defending.
The other 2 Steel Wing combinations are the ones you would want to avoid.

With dragonite, because dps is fairly consistent across its moves, as movesets get worse on offense, they get better on defense. Even the worst possible moveset Dragonite is an amazing defender due to its typing and raw power, not to mention its CP is INSANELY high which of course means it won't get knocked out of gyms as quickly.

yes, but no.
with steel wing, bad attacker but good defender
with dragon breath, bad defender but good attacker