It depends. What’s the character of the parish where you go to mass? Is it generally expected that people come to mass dressed to the nines, or is it more relaxed? The general rule of thumb is not to draw attention to yourself, so would wearing sandals do that? If so, then maybe its best to leave them at home. If not, then it’s not really a big deal.
I would generally caution against flip-flops, mostly because those can make a lot of noise when one is walking up to receive communion and that’s annoying.
But really, I wouldn’t sweat the wardrobe too much. So long as you have a love for the Lord in your heart, and that’s what you focus on at mass, that’s what’s important.