I don't think monotype trainers are inherently easier than trainers without a type specialisation in casual play. If you play on Switch mode (or, in SV, if you don't actively rebel against it being forced on you) then trainers with a diverse roster are at least as easy as type specialists, because you can just pivot into your strongest matchup for free every time you score a KO. A duotype team made with the same design sensibilities Game Freak uses for its monotype teams could end up being even easier to handle, because your primary counter for each type has half as many threats to handle.
The fact that most specialists are pushovers is a consequence of limited AI, overly conservative roster+moveset choices, and the unrivalled power of setup/snowball tactics. Theoretically, monotype teams can present a unique threat, making use of stacked synergies and surprise move choices to break through your one or two conventional checks and put you on the back foot, but the games are too eager to reward the player for basic knowledge of type matchups, so having one Pokemon with SE STAB is almost always enough.
Ultimately, I think one of the big problems with difficulty in Pokemon is that the games don't do enough to make losing feel like a routine part of gameplay. The games are easy, so we expect to win every battle, which means the games can never get noticeably more challenging without making the player feel like a failure if they end up struggling.
A related problem is that there isn't actually very much in-battle strategising in single-player Pokemon, so if you ever do find yourself backed into a corner, there's rarely an opportunity to pivot strategies or mount a comeback outside of heal spam or a game-warping mechanic like Dynamax or affection procs. If the player is caught by surprise (say, because a monotype team had an effective answer for their supposed counter), the eventual loss feels both inevitable and slightly unfair ("how was I supposed to know??"). If Game Freak ever did try and design a truly challenging Pokemon campaign, I think they'd either have to ramp up how much info you could glean about each significant trainer before battling them or they'd have to thoroughly normalise 'losing and trying again' as a pathway to success.
