1- If there are fake bad reviews ruining the company’s reputation, and you are writing good reviews simply to counter those fake reviews and give a realistic and true image of the company, that is permissible because it is not considered deception or lying. However, you cannot lie in the review by saying “I used this product” if you didn’t. Rather, you can say “This product is a great product because…”
2- But if this violates the policies of Trustpilot.com, then it would not be permissible because when you made an account with them, you clicked to agree to accept their policies. Using their service without their approval would be a violation of the contract with them. And many scholars would say that this is not permissible. Yes some scholars say if what you are doing is legal in your country (not illegal) then it’s ok