The question depends on the meaning of legitimacy we adopt.

If we think that legitimacy depends on a democratic vote, a large part of heads of state would not be legitimate; and if we then restrict this concept to a TRUE (but in what sense?) democracy, their number decreases even further.

From a logical point of view, this discussion becomes a vicious circle.

In reality, in politics, someone is considered a head of state if they actually hold power. It is not as if Stalin, Hitler, Napoleon, or Trajan could be considered not heads of state.

Zelensky and Putin are heads of state as long as they effectively hold power in their respective countries; Maduro was head of state as long as he held power, but he no longer is because he is in a U.S. prison.