I have had that salad......and part of "what it is" is the mayo mixed with the jello....sort of a soft seawater green color and like mayo sort of jelled with green jello.
If you cut the mayo in half you will just have chicken salad in green jell-o with mandarin oranges.
I have a recipe for a really good chicken salad, with mayo and lemon, curry powder and celery...without green jell-o....and using either pineapple tidbits or halved grapes mixed in.
Sorry, I don't think there is much in this world that is enhanced by green jello.

I wonder if you would need to cut the lemon juice in half also?

Our family had an old favorite from the 1950's or '60's. We called it green mold. It was a coleslaw with lemon and lime Jell-o to mold it. The Jell-o made it sweet, like the ones with pineapple or whatnot, but it was really good (at least to me, who grew up on it). It was tortured for many years to make it more diet friendly. It had artificial sweetener, lowfat or fat free "mayonnaise", and all the possible substitutions of yoghurt, etc. I won't list all the ways that it was disappointing, but I did lose my taste for it. I found the recipe and made it once (that's how I know that those who didn't grow up on it don't get the appeal). I followed the recipe, as stated, with all the mayonnaise, etc. It was delicious! It was delicious before the Jell-o was added, and that was really just enough to mold it and add that hint of sweet. You couldn't "see" the Jell-o (which you could once it got tinkered with).

Do yourself a favor. Make it the right way, or make a different salad.

If you want something "set", then why not use a nice savoury jelly like a Madeira jelly rather than a sweet dessert jelly? It's not a dish I understand, I have to admit. But, I really enjoy chicken and other veggies "set" in little moulds - mini works of art. Here's an example

https://www.google.com/search?q=images+poulet+en+gélé&rlz=2Y3KTZR_enGB0695GB0695google.manta.mantaray&oq=images+poulet+en+gélé&aqs=chrome..69i57j33.5204j0j4&sourceid=chrome-mobile&ie=UTF-8#imgrc=mr8DuuLU4wSZ1M%3A

And green Jello has that arsenic green color....and that fake artificial flavoring taste.....shudder!

I might try the recipe replacing some of the mayo (dare I use a couple of four letter words?) with Cool Whip....

If you're trying to lighten it up calorie-wise, a cup of mayo has about 1500 calories vs about 200 calories in CW.

But IMO, much worthier calories :-)

I always try to make a first-time recipe exactly as written and if I like it and think it could be improved by substitutions, I'll try them the next time I make it.

Sometimes I'll make a half of a new recipe to try it before serving to guests.

As I have said before....I am blessed with the ability to "imagine flavors" and know enough about cooking and what works to know it a recipe will work as written or needs modification....is is simply not worth making.
Sometimes I will run across a recipe that is just so odd I can't figrue it out....and then I will give it a go as written...one example is that recipe for Kool-aid pickles.
But I feel very comfortable adding some onion or red pepper to a blah recipe or decreasing the amount of sugar....or skipping the green jello in a chicken salad.

Glenda, I've been going under the assumption that this was a recipe you knew and liked and just wanted to make leaner. If it's just a nice presentation you're looking for, however, and you want it to take to a mold, you can do that without the Jell-o, though you loose the volume of the gelatin that bulks up the salad. Steam or poach the chicken so it's very moist and cut it and the other ingredients very small, rather than chunky and use two to three parts chicken to one part veg (you can decorate the platter with more veg to add in on the plate). That's the whole secret. Chopping the chicken fine reveals the stickiness in the protein. It should need a lot less mayonnaise--or just a yoghurt dressing--since you're only sticking the salad together rather than filling the mold with it.

Took the congealed salad to potluck, today, and it was a huge success. I made it as is! I did cut the recipe in half and it was enough for the group.