If you are comparing binary acids then yes the heavier element would be more acidic b/c its base is more stable since it would be able to spread the negative charge around its huge size.
HF and HI = The bond between HF is shorter and stronger so when you put it in water then it wouldn't dissociate that easily. ON the other hand, the bond between HI is longer and weaker since I is large; therefore it would dissociate more easily compared to HF. Therefore, HI is a stronger acid since it dissociates better than HF. You can also compare their bases F- and I-; I- will spread the negative charge around its large size making it more stable therefore its acid would be more reactive or acidic.
Hope this helps..