Low level perfects Pokémon worth stardust?

Not worth the dust... currently. If things change in the future, then the Eevee might be worth it. For the Eevee, depends on your area AND if you're one of the Blessed who both was invited to a Mewtwo raid AND caught one. If you haven't caught one, then it might be worth it to evolve an Esperon.

Regarding the Larvitar, sad to say it's not likely a useful Pokémon to evolve. Waaaaaaaayyyyy too many Candies to evolve it to a Tyranitar, then to power it up to be useful. I'd wait on that until I had both 300 extra Tyranitar candies AND I couldn't catch any more Tyranitars in raids. It's a hell of a lot of candies that would be likely better spent on powering up a 67IV Ttar with Attack 15 than a Level 2 Perfect Larvitar.

Regarding the Mareep? I have no idea.

the same dilemma I have with a 100% lvl 2 Dratini. :cry:

The Larvitar is worth it for sure, and probably the Eevee as well (personally, I'd make it an Espeon). The power up costs from low levels up to level 20 are not that much.

Would you power them up if they were already level 20? If so, then you should power them up from where they are, because it isn't that much different.

It's a difficult question that I know has been talked about here before.

Personally I have a low level 100%IV Cyndaquil, that I've been thinking of evolving and power up, but seeing as Typhlosion isn't the greatest, I've decided to wait.

So I would say that it sort of depends on what Pokemon it is.
Tyranitar is a great Pokemon that will be useful for some time, so if you have the candies, it might be good to evolve your Larvitar and power it up.
According to this sites evolution/power up calculator it would cost 179 candies (it would cost 125 just to evolve it) and 44600 dust to get it to level 20. That's just 54 candies more than it would cost to evolve a hatched one.

Spend 245 candies and about 120000 dust on it and you will have a 3100+ 100%IV Tyranitar at level 30.
Not too bad ;)

As for the other two, I'm not so sure. Ampharos isn't that great, but it wouldn't be that expensive to get it to a lvl 20 Ampharos.

But I would rather spend dust and candies on that Eevee. It wouldn't be too expensive to get a lvl 30 evolved version.

In the end I think you just have to look at what you need, want and can afford.

Tyranitar will remain the best dark type pokemon for many generations, and the only pokemon that will truly surpass it are legendary (and likely to be EX raids so of limited supply, if current EX raids are any indication.) If there is any pokemon worth powering up from level 1, it is Tyranitar.

Anyway, it is "only" 56 candy and 45k stardust from level 1 to 20, then 125 candy to evolve, but after that you can procede without hesitating like you might feel when powering up one with marginally weaker IVs.

I really liked Ampharos in Gold/Silver, but in Go it is far surpassed by Raikou as an attacker (both by stats and because Thundershock is much better than both Ampharos's fast moves).

Larvitar and Vaporeon(if you can get it) worth it!

i'd swap you 200 larvitar candy for your 100% if i could.
I short prophecy
you'll leave him sitting there power up a 93 % one acccumulate 500 candy before you see something better and wish you had of spent the dust on the perfect instead.

I've evolved a perfect Larvitar and Mareep both level 1. The Ampharos I keep as a trophy, but the Tyranitar I have powered up.

The thing with 100% is, you know for a fact you won't obtain a better one in the future. So in that sense, every single stardust is worth it. Besides, from level 1-20 doesn't really require that much extra stardust and candies relatively in my opinion.