While it might sound like a trivial question, the answer is more complicated than it may sound. The current wiki describes what the magical girl genre is about, in slightly broken English, and clarifies that this tag is supposed to be used when characters in the image are wearing a magical girl outfit.
However, the problem with that statement is the sheer diversity of what a given series may consider a Magical Girl. While most people typically associate the genre with pink, frills and ribbons, even as classic of a magical girl series as Bishoujo Senshi Sailor Moon does not strictly conform to that idea, with its simpler sailor-style uniforms. But there are copyrights that might diverge even further - Magia Record's designs are immensely varied from "classic magical girls" to designs such as dominatrix-military style Izumi Kanagi, native American-inspired Himuro Rabi or Mao Himika who wears circus-themed chaps without any pants underneath; and Mahou Shoujo Site's magical girls do not transform fully, rather, their magical girl status is outlined by symbol-shaped pupils, a symbol on their wrist, and bleeding from at least one orifice (see: post #3132840).
Essentially, this tag may be prone to potential canon-tagging. However, in its case, avoiding canon-tagging *may* bring some harm to searchability - a lot of series, such as Mahou Shoujo Madoka Magica or Winx Club do not have separate names for their characters' magical girl persona. Therefore, the best bet for these is to simply search for the MG tag. And even for series with magical girl personas such as Precure, a way to simply search for all magical girls within the copyright at once can be useful.
The potential options are:
- Completely nuking and deprecating it, as was done with goddess - the sheer variety in the genre and designs simply may not validate the tag's existence. May cause problems when searching for general transformed forms of multiple characters/characters within copyrights without special magical girl names.
- Removing non-traditional magical girls, such as aforementioned Izumi Kanagi - leaves only "traditonal" magical girl designs. Defining a hard line between traditional and non-traditional magical girls may be difficult. May still cause problems within no-special-name copyrights.
- Giving non-traditional magical girls a separate tag, similar to nontraditional miko. Still has problems with defining the difference, but searching for all magical girl designs would still be possible via an OR search.
- Leaving matters as-is, with excluding just the most fringe cases like Mahou Shoujo Site where magical girls do not transform, or Magia Record's Swimsuit/Haregi alts, which despite being results of magical transformations, are virtually indistinguishable from normal costumes of such kind (see post #4411760, where the pink and blue ones are transformed but the yellow one isn't). Provides optimal searchability for magical girl characters, along with making a search that would display all sorts of magical girl characters possible, along with requiring minimal effort. May lean heavily on canon-tagging. (Personally leaning the hardest towards this one)
- Accepting *all* magical girls into the tag, including the indistinguishable ones and cases like MSS. I don't see any pluses in it but it technically is an option.