I want to know is there any sin if I cancel a mutah (Engagement) with a girl just because i don't like her looks?

First of all Mut’a is not an engagement but Mut’a has a certain time period so when that time would finish Mut’a marriage will automatically nullify. But it’s not like an engagement because engagement has no Islamic authenticity. Yes you can break Mut’a marriage before it’s time and there is no sin for doing that.
But as you said you want break this marriage just because she doesn’t look beautiful.
No doubt beauty is one of the factor in liking someone but make sure this physical beauty has very short period and then after few years this will go whether you wish or you don’t. But the Akhlaq and character and spiritual beauty will remain until a person is alive. So in marriage we should be very careful that we have to give priority to her/ his Religion, Akhlaq and Behavior. In future she is going to be mother of your kids so if her character is good, she will raise a better generation but if you select a beautiful spouse and her character is not good, then when she will become aged you will loose your attraction and love for her because you have selected her because of her beauty not because of her character, Now her beauty is no more so you will not have any interest in her.
Therefore don’t break this relationship just because she is not so beautiful. Woman is not a model of statue that one has to show her to others and be praised by others as wow you have a nice wife! No thats not Islam.
But if there is another reason and that reason is genuine and really going to affect your martial life in future then you can think about it. But don’t make criteria for your future partner that she must be very beautiful and wealthy etc.
Always look Religion and character of a family where you want to select your wife.
Think carefully and then take any decision you will be responsible for this on the day of Judgment.
But again I’m saying if she has any bad character or behaviour or her family background is not good then you can have choice to accept her or not before having permanent marriage.