This assumes that there is a scientific definition or platonic ideal out there for "dictator". There are many different different definitions, and by some Ataturk is and by some he isn't.

If we think of a dictator as "anyone who is not a democrat and not a royal", then yes, Ataturk was unambiguously a dictator. The period from 1923-1950 is most commonly called the "Early Republican Period", but as I'm sure you know it's also widely known as the "Single Party Period". That right there tells you that this was a rule by a single party. The elections of 1945 were unfair and most people agree that the first competitive elections weren't until 1950, when Ataturk's CHP party (lead by İnönü since Ataturk's death in 1938) lost to the Demokrat Party. Historians increasingly do not see 1923 as the sudden rupture that Turkish historiography has traditionally emphasized. My friend Reuben has a written a neat little piece called, "Ninety Years Since What? Framing the Birth of the Turkish Republic", but the more famous historian Erik Jan Zürcher (the best historian of modern Turkey, for my money) has argued in several places that we should think of 1908-1950 as the "Young Turk Period". It's worth noting that, if we see it like this, the Young Turks came to power in 1908 and retained their power 1919-1923 through military force.

We can either think of these "coups" or "revolutions". The firstwas against an absolutist Sultan. The Ottomans had a Constitution briefly in 1876, but the Sultan abrogated at his first opportunity. The second constitutional period only began when the Young Turks seized power, at their behest. Today, we very much associate the ideas of "government for the people" and "democracy", but in this period, those ideas were not as congruent. Constitutions were seen being more important than elections (though the Young Turks did bring about elections as well, and it wasn't simply a rule by the Triumvirate, as many think). Nader Sohrabi has written the most about this, especially how ideas about how constitutions traveled around the region (see his really neat article, "Global Waves, Local Actors: What the Young Turks Knew About Other Revolutions and Why It Mattered"). It's widely agreed that the Russo-Japanese War, perhaps the first time that a major (modern and constitutional) "oriental" power beat a (backwards and autocratic) European power, was deeply influential on political circles, not least in the the Ottoman Empire and Iran (Russia ended up having a revolution as a result of this loss in 1905, Iran had one in 1906, the Ottomans in 1908). The second coup/revolution was against a Sultan propped a foreign occupier (Britain) and her allies (France, Italy, Greece, Russia, Armenia) who planned on dividing up the Ottoman Empire and leaving Turkey as only a neutered rump state. Ataturk and others used military force to ensure that this did not happen. One only needs to compare the map that the Sultan agreed to at the 1920 Treaty of Sevres and the modern map that was negotiated mostly at the 1923 Treaty of Lausanne, after Ataturk and the national army essentially defeated all of their opponents.

The 1908 Revolution (or coup) brought greater political freedoms that existed before 1908. The 1919-1923 Revolution (or coup) brought greater political freedoms than existed before 1919. Especially after 1923, the government was unambiguously run for the benefit of the people even if there was no democracy. The economy was greatly reformed, which trickled down to common people especially via development banks and factories. "People's Houses" and "Village Institutes" were established that brought education (and CHP ideology) to outlying areas. The Language Reform (called in Turkish "the Language Revolution") brought literacy to the masses, as did the increased public expenditure on schooling. As far as I can tell, by the end of the Early Republican/Single Party/Young Turk period, just about every village had an imam (paid by the state), a primary school teacher, and elected headman (muhtar) as well as an elected "council of elders" (ihtiyar heyeti, though I have yet to find a single instance where the council of elders did very much). There's evidence that Ataturk's party, the CHP (by the 1930's, Ataturk was less involved with day-to-day decision of government--that's something not usually emphasized in the Turkish curriculum) tried very hard to be responsive to the needs of the villagers and the urbanites both but, without voting or a very well-developed bureaucracy in outlying areas, they had very limited ways of actually finding out what "the people" needed (their feedback mechanisms seemed to be at best ad hoc petitions and responses through the People's Houses). Most of the reforms that the people didn't like were ones that we might today still recognize as good ones (especially in the terms of secular government, education, development, and women's rights). Some of the reforms, especially in the realm of religion, did "go too far", i.e. closing all the Islamic institutes of higher learning (reopened by the DP in 1950, I believe), banning all the Islamic brotherhoods (they're still ambiguously banned today, but have been tolerated more or less since the 1970's), and turning the call to prayer into Turkish (changed back to Arabic by Inonu as a voting getting gesture in the lead up to democracy).

Ataturk also notably "took off his uniform" to rule, and ruled as a civilian. This did not happen in Turkey in 1960, 1971, or 1980, and when we think of "dictators", we often think of people whose rule is backed up primarily by the military. Ataturk was not like that. He was immensely popular and I have never heard anyone doubt that if there were an election, he would undoubtedly have won (at least, obviously, initially). That is not to say that his rule was entirely peaceful or that he was universally loved. There numbers are off the top of my head, but there I believe there were 18 major uprisings between 1923 and the Dersim Massacre of 1938, almost all of which (14? 16?) were by Kurds (the remainder were religious rebellions lead by conservatives). The Kurds were divided by religion (Sunni/Alevi), language (Kurmanji/Zazaki), and tribal alliances, and thus none of the rebellions actually managed to unite most Kurds together and all were put down. Some scholars have sought, rightly, to connect the the violence against non-Turks across the entire Young Turk Period (1908-1950), Uğur Ümit Üngör most convincingly, in that this was a clear ideological policy of the Young Turk governments to transform Anatolia into the ethnic homeland of Turks. However, under Ataturk, both the population exchange with Greece and what Üngör calls the "social engineering" of the Kurds, was far, far less brutal than the expulsion and systematic murder of the Ottoman Christians in Eastern Anatolia (commonly called "the Armenian Genocide" outside of Turkey, but involving Armenians, Greeks, Syriacs) that took place under the previous Young Turk government. Still, there was a clear policy that Kurds could only be members of the polity--and therefore enjoy the full benefits of the state--if they gave up their Kurdishness. I am not sure I would single out Ataturk for this, as ethnicity was the basis not only for the Turkish state, but most European nation-states of this period.

So the question of "Was Ataturk a dictator?" largely rests in the eye of the beholder. The type of ruler he was was more common in his period and after. Farheed Zakaria, in a 1997 article, coined the term "illiberal democracy" to describe an emerging form of government (Russia, Singapore) that had elections and some trappings of democracy without the open society, civil liberties, etc. that we associated with democracies (his article proved prescient: we've see so many democracies become increasingly illiberal and so many dictators adopt elections to shore up their legitimacy that that it's sometimes hard to tell difference). If we think of Ataturk as a dictator, we must remember to think of him as a "liberal dictator". He is often compared to Lee Kuan Yew, the first prime minister of independent Singapore, a classic example of an illiberal democracy (Lee was prime minister 1959 to 1990), who strongly developed the country and created some liberal institutions and some autocratic ones.

The most I can say is that while Ataturk was no democrat in practice (see next comment), he did clearly rule for the benefit of the country and the people of country (especially, obviously, the ones who adopted a Turkish-Sunni identity) rather than for personal gain. Often, his rule was characterized by what a particular elite group (mostly Turkish refugees from the Balkans and Turks from Istanbul and the Aegean region, as Erik Jan Zürcher has pointed out) thought was best for the people, not what the people necessarily thought best for themselves. This is fundamentally at odds with democratic values. Compared to his time (the 1920's and 30's were a global nadir of democracy), however, I think it's hard to argue that his government was either particularly illiberal or particularly unjust. There's no simple answer to your question.