Although Froslass may seem appealing due to it's high speed, it's a really mediocre pokemon. Although it's UU by smogon standarts, that's because it's serves as a spikes setter. However, that utility isn't all that useful in Reborn, since no one is changing their Pokemon a lot, and Froslass doesn't hit hard. If you need an ice type, then I can recommend you 4 options:
Ninetales: Although it has almost the same speed and special attack, it can boost it's offenses via Nasty Plot. Snow Warning is an amazing ability, since it makes Blizzard 100% accurate, enables Aurora Veil, and can trigger Slush Rush for Pokemon like Alolan Sandslash. Fairy is also a good offensive and defensive typing, but you'll need to breed Moonblast onto it. Vanilluxe is another good option for a Snow Warning user that can hit harder, but it doesn't have access to Aurora Veil, or a high speed stat.
Mamoswine: I don't think I have to explain this one. Ice + Ground is an astonishingly good offensive typing, hitting at least 9 types super effectively. Coupled with a beefy attack stat, Mamoswine is sure to nab a KO onto almost anything. Thick Fat is the preferred ability, since it helps it check Fire Types far easily. You'll need to breed it Icicle Crash from an Alolan Sandslash, i think
Jynx: It has access to dual STAB in Psychic and Ice, not the best, but it's always useful if you need to diversify your team a bit. Since it has access to Nasty Plot, it can serve as a late game setup sweeper, and it will hit sooo damn hard after just one. It may be PU in Smogon, but that doesn't mean it can't do well in Reborn, thanks to its high speed and special attack
Weavile: Another one that I don't need to explain. Same as Mamoswine, but it has higher speed, and different STAB. Weavile hits hard and fast, and it has access to priority in Ice Shard. Again, you'll need to breed it Icicle Crash via Alolan Sandslash/Sandshrew.
All of them are good options. Choose one that fits onto your team. I like A LOT the utility provided by A-Ninetales, but i may be slightly biased, since it's overall one of my favorite Pokemon ^~^, but i can't deny it's usefulness, but if you need a hard hitter, then DEFINITELY go for the other options, specially Mamoswine.