Regarding Pokémon designs after evolving, I was mostly disappointed with the designs for many Pokémon given new evolutions in generation four, such as Magnezone, Electivire, and Magmortar. Even though I love these three Pokemon for other reasons, I feel that their designs could have been much better. I believe Electrivire should have looked more similar to (but not exactly like) Zeraora instead of the big goofy-looking thing we have now, as it looked more like the direction that Electabuzz would have went to. I feel that Magmortar should have a sleeker appearance, similar to Mewtwo's shape and stature, but still with a fiery body much like Magmar. Magnezone could have easily looked like a sleeker magnet styled warship but instead came out resembling a toy UFO for kids in pre-school without really much similar to Magneton. Stat-wise/battle wise, I guess I would have to say Delcatty. Despite being a final evolution form, Delcatty is still weaker than almost every middle evolution and even some first stage evolutions are stronger such as Abra and so on.
I agree on all of these with you. As regards disappointing Sinnoh evolutions, let me also add Rhyperior, Mismagius, Yanmega, Froslass, Honchkrow, Roserade and Dusknoir to the list. None of them keep the charm of their previous evolutions - Honchkrow even drops the witch themes of its pre-evolution in favor of a mafia don theme! I'm not the biggest fan of Misdreavus or Dusclops either, and even
their evolutions are underwhelming. You also mentioned Delcatty, which in addition to being weak, is in my opinion the worst-designed cat Pokémon on par with Liepard and the Galarian Meowth line. I wish it had been more like Skitty.
As regards Pokémon in other Generations... I'd say my greatest disappointments all come from Gen VIII. First was the Impidimp line. I was expecting Impidimp to become a funny mockup of the Devil from Nintendo's game
Devil World, or at least a gargoyle... but they chose to go completely serious and give it an oni theme of all things for some reason. My first reaction upon seeing Grimmsnarl was, "Look how they massacred my boy." My second greatest disappointment was, as ReKoil mentioned, Boltund. It looks oddly... robotic and creepy, and a far cry from how cute and lifelike Yamper is. Finally, while I do kind of like the concept behind Inteleon, I wish its design looked less disastrously humanoid. I hate how messed up its proportions look.
Across Generations, I could count Venusaur, Machamp, Wigglytuff, Shiftry, Lopunny, the aforementioned Liepard, the elemental monkeys, Delphox, Diggersby, Talonflame, Florges, Gourgeist (;_;), Crabominable, Salazzle, Obstagoon, Galarian Rapidash, and most of the Hisuian evolutions (barring the starters, Wyrdeer, Kleavor and Sneasler.) The three above, though, are the ones I recall best.