I have to strongly disagree. Pokémon is not getting worse simply because I dislike a Pokémon's design. In fact, I honestly couldn't care less about a Pokémon's design when actually using it in battle. I also don't believe that Pokémon is getting worse overall as a franchise, but instead, is getting better and has done so with each passing generation.
Beginning with generation I, despite being the beginning of the Pokémon franchise that introduced us to the video game series that we all love, generation I was an unbalanced glitchy mess of a game. Psychic-type Pokémon were too powerful, moves missed more often, type matchups that were supposed to have worked did not work such as Ghost-type vs. Psychic-type, and so on.
Generation II improved upon generation I by adding some more balance to the game by introducing the Dark and Steel-types, correcting the glitches that caused the type matchups not to work properly, added more features and content such as a day/night cycle, real-time clock and events that occur on certain times, PokéGear, the ability of Pokémon to hold items for battle, the addition of Kanto, along with Johto, and 100 new Pokémon, all of which made Pokémon better.
Next, with generation III, while the game did slightly regress by not including a visible day/night system, generation III changed the landscape for competitive battlers by introducing Pokémon having different abilities in battle, revamped the EV system that gives trainers more freedom to control their Pokémon's stat growth, and so on. The changes in the battle mechanics signified that Pokémon took a step in the right direction and improved, even though generation II had better overall games in my opinion.
Game Freak definitely stepped their game up with generation IV, in which they made changes that changed the game of Pokémon in ways that have become useful staples today. Generation IV determined physical and special attacks based upon the individual move rather than the typing, which made many Pokémon that struggled to be good much better. For example, despite a Pokémon like Gyarados having good stats, Gyarados was never good in games before generation IV because its primary Water typing only dealt special damage and it had a bad Special Attack/Special stat. In addition, it had a great Attack stat, but couldn't make good use of it because it couldn't learn a single move of its secondary Flying typing that did physical damage. Generation IV also provided many Pokémon with much needed evolutions that made them more relevant.
Generation V went in the direction of generation IV and only made the games better. Generation V introduced the most new Pokémon in the series, created a plethora of Pokémon that became mainstays in the competitive scene, introduced the concept of Hidden Abilities, which made many different Pokémon (such as Ditto and the starter Pokémon) much better competitively, introduced fully animated sprites, and put more emphasis on the storytelling aspect of Pokémon. Game Freak also seemed to notice that many Pokémon players weren't all children at this time and seemed to aim generation V more towards teenagers and young adults as opposed to just children like the other games. I believe the changes in generation V really helped the series improve drastically.
Generation VI improved upon the Pokémon franchise by converting Pokémon into a fully 3D game, which greatly improved upon Pokémon's visuals when battling and exploring the vast open worlds of the Kalos and Hoenn regions. Generation VI also introduced battle gimmicks, with started with Mega Evolution, which could give trainers an edge in battle if they use certain Pokémon. Generation VI also introduced the Exp. Share, which made training Pokémon to certain levels much less of a grind. They also created the much needed Fairy-type that added some balance to the game, since Dragon-types were just too powerful, especially Pokémon such as Hydregion and Haxorus.
Generation VII made improvements upon many features in Pokémon that made playing the games more user friendly. For example, generation VII removed HM moves entirely, which allowed players to finally choose whatever moves they wanted for their Pokémon without making them HM slaves. They also took a lot of chances and deviated from the traditional gym badge challenge in a more positive way, as well as introduced many different ways to earn money and items through various side-quests throughout the game. The spin-off games such as the Let's Go series introduced the ability to swap Pokémon from the party and the boxes without necessitating the use of a Pokémon Center PC. The series also introduced wild Pokémon free-roaming in the open world, which is a vast improvement from being randomly attacked in the grass, caves, and bodies of water, which made movement often difficult and annoying.
Lastly, generation VIII improved upon the franchise in a big way as well. They took everything generation VII improved on and upgraded it. They also especially made improvements for the competitive players. Training Pokémon has never been easier, as you level up your Pokémon easily with Exp. Candies that the game generously gives up throughout your play-through. They also made it so that vitamins can completely max a Pokémon's EVs. In addition, the competitive battling scene is probably at its best on a meta level, as the Battle Stadium has ranked battles in both singles and doubles that allow players to achieve master rank, as well as many official competitions with varying rules. Pokémon Legends: Arceus also introduce several new features in Pokémon that made all sorts of Pokémon fans happy, such as deeper RPG elements, pseudo-free roaming, and a new way to catch Pokémon entirely.
Thus, all in all, Pokémon is only getting better. Sure, while each generation might introduce some features that we might dislike, overall, the good definitely outweighs the bad. I am looking forward to seeing what improvements Game Freak made for the generation IX as well.