The idea with the extension names is to have different names for different ABIs. See https://wiki.debian.org/Multiarch/Tuples for the list of ABIs that are "known", or documented.

mips-linux-gnu is documented as MIPS32r2+FPXX, which is a different ABI than MIPS32r2 apparently, and not compatible, so it should have a different name.

Other architectures like arm-linux-gnueabi and arm-linux-gnueabihf have the hard/soft difference encode in their GNU triplet name, and apparently nobody did that for mips.

One solution would be to define a mips-linux-gnusf Multiarch name, which then would be different than the GNU triplet.  Note that there's no "authority" to define the Multiarch names, however in most cases the GNU quadruplet without the vendor is chosen as the MA name.

Other solutions could be to define no MA name, or go with your solution, just ignoring things. I'd still prefer something defining a name for this ABI.