PT is not a deep character, does not have a 'high skill ceiling' and is overrated.
Mad enough yet? Good, now read on.
In fighting games, the idea of having 1 character cover another's bad matchups certainly makes the individual who uses them more likely to succeed, but the characters themselves need to actually be good. The prime example in smash would be brawl zelda/sheik. Individually, zelda was one of the worst characters in the game and sheik was mid-tier at best thanks to getting chaingrabbed to hell and back. The idea of using sheik to rack up damage, and Zelda to KO was incredibly naive because in practice, sheik would just get grabbed all the way up to high % while her attacks didnt do much, and then a swap to zelda to KO was pointless because you were at a % disadvantage, with one of the slowest characters in the game.
Had sheik been a top tier character to begin with, she would have been far more likely to get the enemy to 100% while shes on maybe 50%, and then switch to zelda. That would have made her pretty good. But of course she didnt. That never stopped people continuously rating 'sheilda' as high tier despite never doing anything of note in the games history. It would seem the mistake people were making is that they were adding the characters strengths, and subtracting their weaknesses but that is incredibly flawed since as I mentioned, pretending like Zelda has sheiks speed when it comes to landing a KO move never works out.
PT is the same in this game, however noticeably better than the brawl variant because they are actually pretty good. That said, the individual characters are not deep at all. Consider for a moment, if you had 3 widely agreed not 'deep' characters. Lets say Ganondorf, chrom and kirby could switch between each other mid-match. The 'depth' of the character is not the sum of all 3, its measured by choosing when to use each one in each scenario. But really, is that even hard?
In theory, the highest possible level of play of PT would see people switch to the optimal pokemon at any given time but it is impossible to ever achieve this because that is forgetting one gigantic thing; there are actually 2 players in a match. Say you're up against someone like Snake and you think 'ok its time to zone break him and I'm going to use squirtle'. Cool theory, but now the snake player goes 'fine, no grenades, I'll just trade with normals and hopelessly outdamage you and rely on nade counters literally every time I'm hit.' So the PT player has to adapt. But wait... literally every character in the game has to do that. You adopt a certain playstyle, the enemy changes up theirs, you change yours, they change theirs repeat forever. PT is not unique in the slightest in this regard. Just because you can mix up your approach with 3 different characters, doesn't mean that the character as a whole has more depth, its simply using more characters.
As I said before though, no one would ever imply that a team of ganondorf, chrom and kirby are 'deep', so why with PT? The individual pokemon are quite literally some of the most basic characters in the game. Squirtle and charizard are your generic archetypes that don't need to rely on all sorts of bizarre set ups like megaman, robin, ryu etc. Their neutral is limited to like, 2-3 approach options and thats it. If you are honest with yourself and ask 'how many characters in the game, are more basic than squirtle or charizard' and you might find the answer is pretty small. Ivysaur is more complex because of how many options it has in neutral and edgeguarding, its really got a lot going for it.
Given that the individual pokemon are not very deep, and deciding when to switch to each one requires as much depth as playing literally any other character, the character by definition can not have a high skill ceiling and thus is overrated by people who talk about its 'potential' when played at the highest level. The level that PT is at right now, is the same level that all other characters are sitting at. PT as a character is not going to magically have a ton of room to grow where other characters wont, because the individual characters simply do not have the room to do so. Optimal switching can not exist for as long as the opponent is capable of adapting (see; all top players) therefore its theoretical peak is hard limited.
The final remaining argument is that for someone to compete at a high level with PT, they need to master 3 characters instead of most people who master one. This is basically admitting that the character requires more effort than others, to achieve a comparable level of success. You know what that is known as in all other fighting games? Its called a 'mid tier'. If your character requires 3x200 hours to match your results that you would have got in 200 hours on a top tier character, then how is that different to spending 600 hours on a different mid tier, to get the same results? Hint: Its not.