Recently, someone asked my family for my hand in marriage. This person is a mu'minin. I want to say no. But it is not because he is a mu'minin in fact I was looking to marry someone in the future who is respectful and pious and mindful of God. But there is another reason, it has nothing to do with his faith or job or look or anything like that. I have heard before if a mu'minin asks for your hand in marriage and you reject then you won't really find a mu'minin to marry later or there is no barakah in it. If I say no, does that mean I will not have tawfiq in finding a pious future spouse? I am not rejecting because he is a good person but rather because of another reason.

In general, if a Mo’min proposes one should not reject the proposal unless there is a valid and legitimate reason. Between you and Allah, if you believe the reason is justified, then that’s ok it should not take away tawfiq in finding a pious spouse in the future. The tawfiq is taken away if you’d say no due to materialistic reasons, such as preferring a non-Mo’min over a Mo’min because the non-Mo’min is richer or has a greater social status.
But if you believe the reason is legitimate and you are determined to marry a Mo’min in the future and you’d never prefer a non-Mo’min over a Mo’min then it should not affect your tawfiq Insha’Allah.