Most cards have corner cases when the more powerful are worse than the less powerful. So, no, when a card is most of the time better, then it is often a power crept version. And no, I do not agree that a card has to be in your upper tier and then made stronger to talk about power creep. If the creatures get more powerful, then whole strategies become useless. That could be:
A cube with only a few tier 1 creatures a bit more tier 2 and a lot tier 3. Changing the tier 3 creatures to tier 2 is a creature power creep (could be for the best or not, depends on the wishes) and then a whole slew of strategies could be inviable (and maybe a few new ones emerge).