Sure it would be OK. The point about Presidents not saying bad things about other Presidents applies to PResidents. If Palin wins, then he wouldn’t say bad things about her because she’d be a President.
Bush shouldn’t “bad-mouth” Palin, and there’s no real evidence he did, but the reason for not doing so is entirely different than the reason he won’t say bad things about the current President, even though the current President doesn’t follow the courtesy that most every President has followed.
Presidents should bad mouth current presidents if what they are doing is detrimental to the health of the country. The oath of a President doesn’t stop after he leaves office. He still is sworn to protect the country from all enemies foreign and domestic, that includes sitting Presidents. Taking the “high ground” is what cost this country much of our freedom. Bush needs to have his ass whipped or jailed at the very least for the hurt his BS closed mouth policies have cost this country.