If I hash different seeds with the same size of hash output and XOR on plaintext, is it secure as a One-time pad?
Ignoring the 256-bit blocks specified in the question body, the answer to the question in the title depends on the hash length.
Consider a block size of 1 byte. A hash function with 8-bit output will map each byte into a random other byte. But some of the input bytes will map into the same output - on average there will be 161 distinct output values for the 256 distinct input values. A one-time pad covering only 63% of byte values is clearly not as secure as a proper one-time pad.
At the other extreme, you can consider a hash input block size of twice the message length. At this point, it is almost certain that the hash output after truncation to message length will cover all values close to uniformly, and is equivalent to a random one-time pad.
Where exactly the threshold lies will depend on message contents and how much other information is available for cryptanalysis. Without knowledge of message structure, a one-time pad is not vulnerable to direct brute forcing. Even a very weak pad couldn't be broken if you don't have a way to verify that the decryption is correct. For highly structured and repetitive content, even 256-bit hashing could have enough of a bias to distinguish the encrypted data.