Thucydides was not a happy bunny. Strictly speaking, we don’t know this – even if we trust the ancient biographical accounts, it’s not the sort of thing they talk about – but that has never stopped later readers imagining the personality of the author. In the tradition of ‘realism’, most explicitly in Friedrich Nietzsche’s account in Götzendämmerung but pervading many 20th-century political readings, Thucydides is presented as the sort of illusionless man who has the courage to face unvarnished reality; this reading is based on his stripping away of claims about justice and virtue to reveal the power struggles underneath, and then in turn this conception of his ruthless critical spirit is taken as a guarantee of the veracity of his account of the world. Arnold Toynbee in contrast detects an anguished, traumatised figure between the lines of his tightly controlled analytical prose, someone who was broken by his experience of failure, exile and defeat but put himself back together through sheer will and intellectual rigour. In either case, this is not a man who made balloon animals.
But of course we can’t know this; this is all projection, imagining the Thucydides that we want as reassuring authority figure and guru, taking his own (minimal) self-representation at face value and turning it up to eleven. He had a wife, and apparently at least one child; who’s to say that he wasn’t, at least by Athenian standards, a loving husband and father, a loyal friend, a cheerful companion who liked a drink or two?
This is the (marginally) more serious aspect of thinking about the Happy Thucydides, following the work of Martin O’Neill from Politics in York to apply FaceApp to portraits of different philosophers and thus create a Happy History of Western Philosophy (see also the hilarious captions added by Kieran Healy on the Twitter). We can imagine suitable rewrites of famous quotes from Thucydides’ work – “the secret of happiness is freedom, the secret of freedom is happiness, man”; “the strong do what they want? Let’s everybody do what they want!” – but also wonder what difference it would make to our reading of the work is we have this image of its author in mind.
How much do we need an idea of Thucydides as eternally serious and engaged with the world as it is, as someone we can trust and always turn to for enlightenment – but who actually relieves us of the need to be serious and critical all the time? Thucydides can be illusionless and anguished on our behalf – and the effect of this would be less if we had to think of him as someone less consistent, more subject to other emotions, more complicated, less in control.
I now feel the urge to research the theme of Thucydidean jokes.
Sphinx 
You – or someone – has probably done this, but if one was to attempt to asses the degree of distortion in our “reception” of Thucydides, whom we know only from his work, wouldn’t you compare him to multiply-attested figures, like the dramatists, who also show up in Plato and Aristophanes?
Interesting idea, but my immediate reaction is that the situations are very different. Yes, we get an image of the personalities of the dramatists (e.g. in Frogs, or Aristophanes in the Symposium), but these are not important for interpreting their work – no one thinks that Aristophanes’ caricature is important for understanding Aeschylus, so far as I know. The closest we get is the idea of Euripides as misogynist and questioner of traditional values, but we don’t actually need Aristophanes for that. The problem with Thucydides is that his imagined character is sometimes evoked as evidence to support a certain reading of his work as reliable and insightful – perhaps because the question of its relation to reality is much more pressing than with the dramatists. That is: with e.g. Sophocles we’re interested in literary skill, imagination, ideas etc that can be read from the text rather than requiring external evidence; with Thucydides we’re interested in whether he offers a reliable account of external events and/or a valid analysis of the way the world works, so it’s always about the relation between his text and the world, never just about the text, and so other evidence that appears to tell us about how he would relate text and world appears useful.