I have seen so many episodes of Pokemon. Many episodes have Team Rocket trying to steal Ash's Pokemon by disguising themselves, or with other predicable methods. Still, they are easily recognizable as Team Rocket every time. Why are Ash and his friends fooled every time, when Team Rocket's actions should be predictable by now?
It's not that Ash and co. are constantly fooled by Team Rocket again and again, but rather that Team Rocket has a specific role in the show. In order to keep Team Rocket in the show, they have to have a purpose for each episode, and thus have been constantly after Ash's Pikachu since the original few episodes. To us, it seems that Ash is "dumb" or oblivious to Team Rocket's schemes, but in all reality it is due to the writing staff trying to keep them in the show (you have to remember that Pokemon is considered to be a children's show).
I found from here one reason for why Team Rocket still appears in Pokemon:
Because they are popular comic relief characters, and if they were ever to get rid of them, a boatload of rocket shipping fangirls would freak out and hate Nintendo and stop buying games and watching shows, therefore bankrupting the company. How else would they manage to make those episodes last 22 minutes without wasting our time with another predictable plot to capture Pikachu?
At this point, Team Rocket has become a staple of the Pokemon universe, and it would feel empty without them. There are other Pokemon watchers who feel similarly to you, as stated here:
The comic relief they provide is a necessary part of the show, but I'm quite certain they could think of better ways of working it in. Not quite the same as how you feel about strangling them, but my brother has actually started cheering them on because he wants them to actually win for once so that they aren't completely irrelevant. Plus it would be fun to see Ash totally lose it when Pikachu is gone for days on end ;) The thing that really bugs me though is how they have no concept of how you're supposed to pull off crimes anyway. I can think of several instances off the top of my head where if they'd kept their traps shut (no pun intended) without constantly going off into their motto that they might actually have succeeded. They also think that as soon as they've got a net around something that it's theirs. Team Rocket is always shocked when the Pokémon fight back. Finally, who the **** travels in a hot-air balloon when you're trying to be inconspicuous?!
One way to understand this is that the disguises are made obvious for the viewers, so that we are always aware that it is Team Rocket. From an in-universe perspective, the disguises must be fairly effective, because they always fool everyone.
From TV tropes:
The external reason for the flimsy disguise may be that the creators want to signal the presence of a disguise to the audience before the other characters catch on (a sort of Reverse Whodunnit).
Another point to consider is that this is a cartoon. In the real world, it doesn't take much alteration to one's appearance to make it hard for other people to identify you. But in a cartoon, where detail is lower, it's a lot easier to pick out characters so long as the alterations are minor and their features are prominent enough (such as having giant purple or red hair). If we consider Ash and friends seeing things more like we see them in the real world, and less like the cartoon world we see them in, then this makes a bit more sense (Jesse's hair is still pretty distinctive, though).
Some sources on how easy it can be to fool people with simple disguises:
https://psycnet.apa.org/doiLanding?doi=10.1037%2Fxap0000213 (scholarly article mentioned by above gizmodo article, access required)