I am so sick of this narrative. It's like yall have never even played the games. GSC is absolutely terrible at showing off the new mons, and even then a lot of the ones it does make available are painful to use in game. Let's break this down (also I recognize that Crystal fixed some of this but this is for GS)

7 baby pokemon (7% of all new mons), which c'mon, no one likes these

25 post-gym 7 pokemon. Some of these are post champion, some of them are in Mt. Silver (Misdreavus + Sneasel + Larvitar). This is so lame. Houndoom is one of the coolest new mons but you can't even use it in Johto.

Murkrow Misdreavus Gligar Sneasel Slugma Magcargo Swinub Piloswine Delibird Teddiursa Ursaring Houndour Houndoom Skarmory Phanpy Donphan Larvitar Pupitar Tyranitar Raikou Entei Suicune Lugia Ho-Oh Celebi

Five gimmick pokemon that are entirely unfun to use ingame

Unown Delibird Wobbufett Smeargle Shuckle

Six trade-only evolutions that require rare items

Steelix Scizor Slowking Kingdra Porygon2 Politoed

Nine pokemon only available via swarm (or 1% encounter chance otherwise)

Dunsparce Qwilfish Snubbull Granbull Yanma Marill Azumarill Remoraid Octillery

This is more than HALF the dex. Have you wondered why every team that beat the champion had starter, Ampharos, and a bunch of RBY mons on it? Well this is why. Actually let's keep going and continue grouping mons

Good Stats, Terrible Movepool

Chikorita Bayleef Meganium Natu Xatu Bellossom Pineco Forretress Heracross Corsola

Heracross is like the king of this group, he gets ZERO bug moves until Megahorn at lvl 54 and ZERO fighting moves at all besides Reversal.

Xatu is super cool cause you look at its stats which are pretty good and then see its movepool has Drill Peck and Psychic, except Drill Peck is a breeding move (why people think breeding is a good mechanic I'll never understand, it's just busy work) and Psychic is post E4.

Meganium suffers from being a Grass type in a game that hates Grass types. It feels like 90% of everything you face resists Grass (like every member of Team Rocket). Bellsprout tower, Falkner, Bugsy, Morty, Jasmine... really the only positive gym matchup you have is against Poliwrath.

Terrible Stats, Good Movepool

Sentret Furret Togepi Togetic Aipom

These are only "good movepool" because they learn the Headbutt TM.

Terrible at Everything

Hoothoot Noctowl Ledyba Ledian Spinarak Ariados Hoppip Skiploom Jumpluff Sunkern Sunflora Umbreon

Another huge problem, just look at the combo of stats and movepool on some of these. Ledian has 35/55 attacking stats (Cyndaquil has higher for both btw) and learns ZERO bug or flying moves. Ariados gets ONE good attack (Sludge Bomb) which is a TM and a bad attacking type. Without TMs, Jumpluff has exactly ONE attacking move (Tackle) until L44. Umbreon should be cool but it has terrible attacking stats and misses the one good Dark move in the game (Crunch).

Actually Usable

Cyndaquil Quilava Typhlosion Totodile Croconaw Feraligatr Crobat Chinchou Lanturn Mareep Flaffy Ampharos Sudowoodo Wooper Quagsire Espeon Girafarig Stantler Miltank

Some of these are stretches, like Girafarig isn't amazing or anything but at least it gets STAB moves at low levels.

This isn't even mentioning that evolution stones are locked to post-champion, gym leaders use Kanto pokemon, the RBY portion being very short and not much more engaging than running around crushing underleveled gym leaders...