It's adorable.
The description seems a little small, though functional.

D'aww! More little guys are always welcome.

Does its diet change in the winter relative to the summer? I don't think all flora in its diet go dormant. Does it gorge on food, accumulating a layer of fat, takes and stores food, or some combination of both? Does it still have sexual display tufts like its ancestor?

Over winter I guess it sorta becomes a detritivore, since it eats the frozen remains of summer flora? Not sure how I'll describe that though.

The fluff completely overruns its tufts.

If it breeds up to six times a year, but the winters would logically be long and harsh...is "six times a year" only in lab conditions? Judging by the length of the warm season in Barrow, Alaska, used as a proxy for the tundra in general, if it uses only the warm season proper, assuming a weaning age of 28 days (the standard for rats), it should be able to fit in 3.64 batches if maximally productive, or 4.8 if weaned at 21 days, the standard for lab rats specifically. Unless, of course, it can do the same trick kangaroos do and can have one baby out of the pouch nearly mature, another in the pouch, and another embryo paused inside her body.
Does the fluff overrun its tufts even in summer?

Approval Checklist:
Art:
Art Present?: Y
Art clear?: Y
Gen number?: Y
All limbs shown?: Y
Reasonably Comparable to Ancestor?: Y
Realistic additions?: Y

Name: Snowtunnel Shrew
Binomial Taxonomic Name?: Geminatisorex glaciperfossor
Creator?: Disgustedorite

Ancestor: Pikashrew (No, not Pikachu)
Listed?: Y
What changes?:
  • External?: White long fur, though it does shed to black fur in the summer
  • Internal?: N
  • Behavioral/Mental?: More social and burrows underground
Are Changes Realistic?: Y
New Genus Needed?: N

Habitat:
Type?: 1/2
Flavor?: 3/3
Connected?: Y
Wildcard?: N

Size: 13 cm Long
Same as Ancestor?: Y
Within range?: N/A
Exception?:

Support: Endoskeleton
Same as Ancestor?: Y (Though that will have to be added to the Pikashrew shortly)
Reasonable changes (if any)?: N/A
Other?:

Diet: Herbivore
Same as Ancestor?: Mostly
Transition Rule?: Y
Reasonable changes (if any)?: Lost scavenging habit

Respiration: Active (Lungs)
Same as Ancestor?: Y
Does It Fit Habitat?: Y
Reasonable changes (if any)?:
Other?:

Thermoregulation: Endotherm (Fur)
Same as Ancestor?: Does a butterfly have wings?
Does It Fit Habitat?: Y
Reasonable changes (if any)?:
Other?:

Reproduction: Sexual (male and female, live birth, pouch and milk)
Same as Ancestor?: Y
Does It Fit Habitat?: Y
Reasonable changes (if any)?:
Other?:

Description:
Length?: This might be the shortest description so far?
Capitalized correctly?: Y
Replace/Split from ancestor?: Split
Other?:

Status: Approved (Despite how short the description is, it is fair enough for explaining exactly what needs to be explained, but try to avoid doing this again, please.)