#NotALawyer
Is it morally wrong? I'll say it's kind of a gray area, and not really share an opinion
Is it potentially unlawful (with regards to US copyright) and open you to legal liabilities? Almost absolutely yes, unless your resultant work is actually fair use. (Unless it's a parody or critique, it probably isn't fair use.)
Is an author going to get sued for running a tip jar on a fanfic? I'd say the odds are pretty low, but it depends on how visible/popular your fanfic is.
So let's kill some pretty big misconceptions:
The "I'm only making a couple hundred bucks, it isn't worth it for X to sue me" rationality (Part 1):This may be true for most civil cases, in which a claimant can usually only sue for actual (provable) damages. US copyright law is one of the few circumstances in US civil law in which a claimant can instead request statutory damages. In layman's terms, instead of suing you for the value for a fixed value
greater than the amount you potentially make based on the nature of the infringement. This can be between $750 and $30,000 per work with
willful infringement resulting in damages up to $150,000, all at the court's discretion. Additionally, at the end of 2020 a law was passed that effectively creates a small claims court for copyright infringement. There are some questions on its constitutionality in terms of the way it was organized, but it likely reduces the expenses of attempting to file a claim against smaller infringers. This may encourage copyright holders to challenge smaller potential infringers, as doing so will be much cheaper than filing in federal court.
$100, part 2: Just because the cost of a case may exceed the payout does not always shield you from a lawsuit. There may be other factors at play. One might be that they follow through on a lawsuit in order to demonstrate that they control and protect their copyright. This can help the author in other instances of infringement for larger, more valuable infringers. Another reason might just be spite--the copyright holder is a litigious bastard and likes to sue people.
"It's fair use!": It probably isn't. Fair use determinations are extremely subjective, and at the mercy of whichever judge it comes before. Fair use gets tested against four factors. Read
https://fairuse.stanford.edu/overview/fair-use/four-factors/ for a more in depth and authoritative source on this. By accepting payment for the potentially infringing work it becomes commercial in nature, which will help to weigh against a fair use claim.
"It's transformative!" (fair use pt.2): It probably isn't, or at least is not transformative enough. A simple fanfic that just takes existing characters and settings to tell a new story is generally not sufficiently transformative. This is typically the most important factor in a fair use defense and is also the hardest to define. The bar of being 'transformative enough' is nebulous, subjective, and particularly sensitive to the opinions of the judge.
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