X.509 ECDSA signature field in DER format includes '00' between bit string and sequence of r and s
No, the 00 simply indicates how many least significant bits are not used in the last octet of the BIT STRING.

As the bit string contains an octet string (consisting of the sequence of two integers) it will always be zero, as all the bits in the BIT STRING are used. This is defined for the BER/DER encoding of BIT STRING itself.

As for why the signature is not specified as an OCTET STRING itself, well, beats me, as most signatures are defines as OCTET STRING anyway, even if they are not constructed such as RSA signatures.