Recent got a 100% IV Muk and 87% IV Porygon 2

the Muk is worth it, Infestation builds energy pretty fast and is useful against bug-weak pkmn
...oh wait it's Muk...probably just a trophy FOR NOW so it's not a good use of stardust imo.
I have my 93% PoisonJab GunkShot still at level 20 and ~130 candies

Porygon 2 is not a final evolved form, so no.

Gunk Shot is better than Sludge Wave.

Porygon2 can be used as a normal grass attacker, excluding the Rhydon match-up.

If his cp fits into your gyms, power him up. He will add diversity to the gyms.

every 100% is worth maxing just becouse there wont be better one and about moves i think one day you will jave change possibility so dont worry about them. i max more or less all 100% ones if i have enough candies and dust