I actually do think the idea is rather solid, numbers-wise. I don't think it'll be used in it's upped form, but I see situations where it can be strong unupped.
Let's compare to say, Fallen Elf, which fits into the niche of cave bear. Let's also consider things like otyugh, squid, etc. for more numbers.
This thing kills anything with >3HP (although you can save it with HP buffs). A very notable card that runs on 3HP is Shrieker, which also happens to be a core part of an extremely strong deck, Grabbow. Even if it doesn't kill, you are essentially freezing a creature for 3 turns (trading 3 extra atk for 3 turns of no attack is perfectly fine.)
Fallen Elf is a 4
3/2 that has a 1
"unreliable" CC/Buff. Cave Bear is a 3/5 5
3 turns of confirmed CC or a very bad buff (but good for stopping antimatter, sosacs, these are unlikely situations because sosac is banned but meh). Just looking at these two cards, Cave Bear is much bulkier and has a more reliable form of CC. While it doesn't past the 5HP mark of being immune to Lightning, it is much bulkier than it's counterpart.
Other CC creeps: Squid, Maxwell, Otyugh. Numbers-wise, Cave Bear gives the most value for stats. It has the stat-level of an upped card (Maxwell's Demon) and it is perfectly arguable that Cave Bear has a better effect since it can delay a creep for 3 turns regardless of its statline but also kills low HP ones vs Paradox.
Squid: Cave Bear has cheaper and is more likely to survive. The effect can be argued as the same or even better.
Otyugh: 4
0/3 with devour. Unfortunately it is hard to compare the 2 cards, but Cave Bear definitely prevails math-wise. And I also happen to prefer maths (also SoP is out in most unupped metas so Oty unfortunately loses one of its best combos).
As you can see Cave Bear is a very strong card. Water/Earth having better CC is a moot point. Deal 3 damage over 3 turns of delay is essentially a repeatable firebolt with freeze and no other card offers that. It also happens to be repeatable, and very much cheaper to play than nymphs with an even cheaper activation cost.