I'll put a hashtag next to the ones I use often and a asterisk next to ones I barely use

Jack Frost
overused
•frosty (why??? it's basically the same as his last name)
•Frost# (all the time tbh)
•snowman (I can see why you would this but damn think of some other names)
•Frostbite (refer to above)
•Jackson* (it's his real name I believe correct me if I'm wrong)
underused
•Popsicle* (guys this name is fucking hilarious)
•Ice cream (guys)
Merida DunBroch
overused
•red* (Jack only b/c let's be real he's the only one who would call her that)
•meri* (hiccup only. I feel in mericcup he uses that a lot)
•mer# (come on)
underused
•mare* (Rapunzel calling her this b/c how much time she spends with angus and horses)
•firefists* (I use this in a hs AU b/c I headcanon on the first day she got so mad she punched a kid and gave him a bloody nose)
•DunBroch# (I barely ever see people call her by her last name?? but I feel like people would do it all the time?? especially in hs AU)
Rapunzel Corona
overused
•punz* (a just-Jack thing)
•Punzie# (come on)
•my flower (um no that sounds creepy af)
underused
•sunshine* (why??isn't??this??used?? more??often??) (no but seriously it's perfect for her.)
•honey/hon* (I feel like her and Merida would have pet names for each other and sometimes they're romantic-y ones like this)
•smiley (guys come on)
Hiccup Haddock
overused
•hic# (um hello?)
•hickey (nah)
•dragon boy* (totally a romantic-y joke that would happen between him and Merida/Rapunzel/Anna/whoever honestly)
underused
(he actually doesn't have that many underused nicknames that I know of but comment any you know.)

I will update this list as Time goes on and will add more characters and nicknames.