And speaking of universal heuristic, in my experience, the quality and relevance of content is strongly inversely correlated with the quality of design. The prettiest websites out there are ones that deliver negative value. The best designed (according to modern trends) user interfaces are the ones with worst ergonomy, wasting user time the most.
And yes, the subset of spam that's most recognized - ED pills, reproductive organ enlargement, members of royalty looking for help managing their finances - they tend to be very simply designed. But their distinguishing feature isn't simplicity of design. It's the carelessness. Typos, bad grammar, highly visible formatting mistakes, etc. When, on occasion, one of that "old school" spam messages tries to pose as a legit transactional e-mail, you can see through the deception by noticing the carelessness in replicating the design of the company being impersonated.
This is just plain untrue, and seems to color your impression of email as well. See e.g. Delta vs Southwest (more polished look is more ergonomic site/app), or New Relic vs Datadog public-facing sites (indistinguishable in "quality" of the design, for competing products where IMO Datadog is better).
What are some examples that put the inverse correlation in your mind?
"Nicely designed" emails are things like flight confirmations, invoices, shipping notices, and that kind of transactional email. The common thread is that some entity with Real Money To Spend on a graphic designer is sending a (usually form) email to lots of people. Most spam outfits do not have those kinds of resources, especially since they need to change up their emails constantly to avoid spam filters.
The closest I would say that the "pretty" stuff gets to spam is newsletters and particularly political fundraising emails; I'm on a lot of political party lists after spending copious college free time working on campaigns.