No, you should NOT use Eviolite Dipplin over Hydrapple in Gen 9 UU
Hydrapple is better than Dipplin in UU, with Hydrapple being A- on the UU Viability Rankings, while Dipplin isn't on it, with Dipplin being in ZU.
Dipplin
Sure, on paper, the extra bulk from Eviolite is nice, with it being able to keep its Eviolite with Sticky Hold. However, in practice, Dipplin is just... passive. It's movepool isn't exactly the best, having little coverage options or utility moves to make use of its bulk. The shallow movepool ends up making Dipplin a sitting duck (or rotting apple in this case..) who ends up being outclassed by its evolved form, Hydrapple, as well as Amoonguss for a defensive Grass-type.
Hydrapple
Hydrapple has a much better movepool and a great Ability in Regenerator, allowing it to have a little HP recovery every time it switches out. Having Nasty Plot to boost the power of Hydrapple's attacks such as Draco Meteor, Fickle Beam, Giga Drain, and Earth Power makes it not passive while having good bulk. (Dipplin and Hydrapple have the same defense and special defense stat, with only their HP being different.) Hydrapple's bulk and offensive power allow it to offer something to teams that Eviolite Dipplin wouldn't be able to.
Also, guess I'll cover this:
"Also if you want a bulky grass, why don't you use Amoonguss?
commented 4 days ago by sumwun"
Unironically, Hydrapple is the better Regenerator Grass-type in UU. Amoonguss is B rank on the UU VR. I asked about Amoonguss' placement on the VR and the reason why Hydrapple is in A- rank as the better Regenerator Grass-type. You can read the response here. The TL;DR of Amoonguss not being better than Hydrapple is losing Spore due to the Sleep Moves ban done by OU and Pokemon who aren't bothered by Amoonguss such as Latios, Tornadus-T, and Zapdos being common.
TL;DR Eviolite Dipplin shouldn't be used over Hydrapple in Gen 9 UU due to it being passive and not having a good movepool for offensive capabilities or being a defensive Pokemon. Hydrapple is better due to it having good bulk, a great Ability in Regenerator, and a good offeensive movepool with Nasty Plot to make it not passive.
Hope this helps!
