There is some evidence for homothetic preference amongst large number of countries, although there are for sure bound to be cases that violate this assumption.
For example, Choi & Krishna (2004), show indirect empirical evidence for this. As the authors report (my comments in )
The restrictions [including homothetic preferences] implied by the theory for
overall (i.e., bidirectional) bilateral trade flows are satisfied for the vast
majority of country pairs in our sample. Having said this, we must note
that in many cases, the theory is only “just” satisfied. Nevertheless, this
finding stands in strong contrast with many previous tests of the theory
that were conducted under the restrictive assumptions of identical factor
prices and identical homothetic preferences
The authors do this by seeing if the estimations from unrestricted models are in line with what the models with the restrictions would predict. Since they dont find much significant differences it stands to reason that this implies the restrictions are valid.