Unfortunately WMP writes its "Mood" tag to a private ID3v2.3 frame. WMP uses the string "VM/Mood" as an identifier.
Here is quote from
www.id3.org/id3v2.3.0 -> Private frame:
4.28. Private frame
This frame is used to contain information from a software producer that its program uses and does not fit into the other frames. The frame consists of an 'Owner identifier' string and the binary data. The 'Owner identifier' is a null-terminated string with a URL containing an email address, or a link to a location where an email address can be found, that belongs to the organisation responsible for the frame. Questions regarding the frame should be sent to the indicated email address. The tag may contain more than one "PRIV" frame but only with different contents. It is recommended to keep the number of "PRIV" frames as low as possible.
<Header for 'Private frame', ID: "PRIV">
Owner identifier <text string> $00
The private data <binary data>
It is not possible for a program developer to create tag reader that would support any arbitrary PRIV frame because the data structure inside such frames is not defined by the ID3v2.3 standard.
I tried my favorite (in addition to MC) tagger program Mp3tag, but it wasn't able to read a PRIV VM/Mood frame either. (The problem I explained above is mentioned also
at the Mp3tag user forum).
In order to support PRIV VM/Mood a developer would need to specifically study files that contain those tags and create dedicated code that can read them.
However, I found a workaround that you can use for permanently changing the files if you just want to preserve the tag values and don't need them in WMP anymore:
Tag & Rename supports this tag. Apparently its developer has taken the steps that are needed to add PRIV WM/Mood support. I am not recommending T&R in general (IMHO, the free Mp3tag program is better if any tagger outside MC is needed), but it can resolve this specific problem. I tested its "Swap Tag Fields" tool. I swapped "Mood" with "Label". T&R stores the label tag in the TPUB ID3v2.3 frame. MC can read TPUB. The value shows up in the Publisher library field. Inside MC you can move the values to your preferred field.