I have recently discovered the trial tag. The questions in it all appear to low quality. Is it worth retiring this tag?
I'd say yes, but not because of the reason you gave.
My support is based on the absence of a common topic as opposed to just a motif. For example:
Receive encrypted string from server
How is this question related to "trial"? Even if the asking user's purpose had something to do with the trial software, we'd expect an answer that can treat the subject in general.
Change Windows Activation/License back to a trial
I'd say the subject is Windows licensing and windows-activation does it nicely.
Converting Windows 8.x Enterprise into Pro so I could activate it and use at home
This has nothing to do with the trial altogether.
Let ask ourselves: Optimally, what are Super User questions that can be categorized as "trial"?
The answer is: Those that entail treatment of trial software or software piracy. Unfortunately, both go against Super User policy. In that one is the kind of question that lose value quickly and another is the forbidden question. Licensing law question do not have these problem but don't belong to Super User in the first place.