cuckolding is specifically about a sexual act. Polyamory is about relationships that can include sex.
Cuckolding has one very specific dynamic (the husband being "forced" to watch another man "take" his wife), whereas with polyamory the dynamics can be far more varied.
The goal with cuckolding is to either screw another man's wife while he watches, to be screwed by another guy while your husband watches, or to watch another guy screw your wife. Polyamory is about having a life, encouraging and nurturing growth, taking responsibility for your decisions and relationships, being open and honest with everyone around you.... while having great sex.
I would suggest, therefore, that they are in no way similar.