Methylcobalamin is good for anything neurological (like neuropathy) and is more biologically available than cyanocobalamin so while I've not noticed that it helps lower blood glucose, supplementing Methyl B12 is certainly beneficial for all cats with health problems. One of my diabetics developed a head twitch and it helped him, and my really old cats just seem to feel better, improved appetite and such. I also use Vitacost and just mix it in with food.
My vet said that most older cats are deficient in B12 as well as those with certain conditions like kidney disease and diabetes who tend to urinate more than usual. A study done by the Winn Feline Foundation agrees.
Vitamin B12 deficiency is a common finding in cats with gastrointestinal diseases, including inflammatory bowel disease, exocrine pancreatic insufficiency, pancreatitis, and small intestinal lymphoma. While the mechanisms behind the associated deficiency in cats are not completely understood, there is strong evidence to support the need for supplementation.
https://www.stevedalepetworld.com/blog/oral-cobalamin-vitamin-b12-better-choice-injectable-cats/
Tanya's CRF site has some good information about the benefits of B vitamins.
https://felinecrf.org/vitamin_b.htm