Since he’s only a Series 1 character, I’m going to try and narrow this down to just Series 1, but it holds for the other series as well. The reason I don’t like Adam all boils down to what Adam is meant to represent. He’s Rose’s foil character: he’s well-educated, successful (career-wise), and most likely well-off monetarily, while Rose only got her GCSE, works in a shop and is generally viewed as poor. However, he’s meant to show that just because you have a better CV than someone else does not mean that you’re better than them.
Money has always been treated as unimportant in Doctor Who. How much money you have or don’t have has no merit when contemplating how good of a person/companion you are (the Doctor himself rarely carries around money). Rose, who grew up lower-middle-class at best, was never shown as particularly preoccupied with money. She travelled with the Doctor because she loves it: the adventure, the new planets and cultures and experiences., etc. Even if she had been one of the companions who had originally known that travelling with the Doctor wasn’t permanent (like Rory, for example), she wouldn’t have tried to use the opportunity to extort money from the situation. She would have left the TARDIS, better for having known the Doctor and content with her experiences (just like Martha, Rory, Amy, Mickey, Jack, Sarah Jane, and every other companion did).
Adam, however, did want something more. For all his education and brilliance, his purpose for being on the TARDIS boiled down to money. He wanted to use time travel to find information about the future and profit from it, completely ignoring how messed up it would have been to steal the technology and market it as his own (when it clearly wasn’t him who came up with it) and how colossally dangerous it would have been for the UNIVERSE if he went to the past with information from the future and ruined the natural timeline for the human race.
Both Rose and Adam are flawed and distinctly human; however, while Rose’s humanity displays itself in her vulnerability and her courage to do what she believes is right in the face of danger, Adam’s humanity shows in the desire for wealth and power and an irresponsibility when dealing with the bigger picture.
So essentially, I don’t like Adam Mitchell because he is not the best. He’s not even close. He forwent the opportunity to see all of time and space just to get a bigger check at the end of the day, and he almost killed The Doctor and Rose in the process.