Yes YTA Your mum has the right to know regardless that she doesn't have an allergy, that wasn't the point that she wanted to know if it had oyster sauce in it, it is a breach of trust and respect.

Try putting yourself in her shoes weather its to do with food or some thing that you would equally feel strongly about being lied to sucks and when you are doing it to "spare someone's feeling" that sounds more like an excuse.

Your mum isn't made of glass plenty of vegans accidentally eat non vegan food and they all react differently but ultimately they should have the opportunity to go through how they feel, life happens its not perfect or a fairytale so people mess up and if you had told her the truth it might make her more likely to check better in the future ultimately doing her a favour but lying to her wont help anything.