Well, ugly is as ugly does:
I say string theory is ugly in the sense that mathematical elegance and physical elegance are two different things. A mathematically elegant theory is simple on paper, but it may lead to complex and confusing predictions that aren’t observed in real experiments. A physically elegant theory is one that explains the data in as simple a way as possible no matter how complex the math needs to be and no matter how many experimental parameters have to be inserted. (I say explains the data not predicts the data because I don’t claim to be a logical positivist. A theory that makes good empirical predictions but tells you nothing deeper is only a useful tool waiting for an explanation. I do not see the Standard Model as such a theory, albeit it is abstract.)
From this perspective, the Standard Model is mathematically troubling but physically very elegant. It explains exactly what we see in experiments, and, while it is confounding to many physicists that it predicts no more, there is some elegance in that. After all, it does explain the world we live in. Since the Higgs boson was discovered in 2012, the Standard Model’s overall predictions have been confirmed to astounding degree of accuracy.Tim Andersen, “Our universe’s fine tuning may be why the Standard Model is so mathematically ugly” at Medium
Of string theory, he says,
String theory has been, in many ways, a dream for a mathematically elegant description of nature where experimental parameters are predicted rather than measured, and the laws of the world we live in become, not one set of many possibilities, but the only possible ones.Tim Andersen, “Our universe’s fine tuning may be why the Standard Model is so mathematically ugly” at Medium
Hat tip: Philip Cunningham
See also: What becomes of science when the evidence does not matter?
Post-modern physics: String theory gets over the need for evidence