Is Chip 'n Dale: Rescue Rangers a spinoff of The Rescuers?

Yes. But no.

From TV Tropes:

Originally, the series was going to star Bernard and Bianca from The Rescuers, which would likely have departed from the books in favor of original content. However, when The Rescuers Down Under was greenlit for production, the series was extensively retooled. In the revised idea from series creator Tad Stones called Metro Mice, the show would center around a new team of heroic animals, which included a chameleon, a baseball-obsessed cricket, a far-sighted eagle, an earlier draft of Gadget, and a mouse that was like Monterey Jack, but had a different name. The main character, though, was an Indiana Jones-type mouse named Kit Colby who sported a fedora and a fluffy collared World War II bomber pilot jacket. When he proposed the show in a meeting with Michael Eisner and Jeffrey Katzenberg, the idea was well-received...except for the character of Kit Colby. At Eisner's suggestion, he was replaced with the chipmunk duo to give the show some established Disney characters to work with. The eagle, cricket and chameleon were dropped, and a fly was added to the mix.

It started as a Rescuers cartoon, then a generic hero team, and then Chip and Dale, existing Disney characters tacked on to round out the Disney updates of old property like Duck Tales and another show. Chip & Dale have been around since the 50s, mostly just pissing on Pluto or Donald, decades before the first Rescuers.

From an old Chip and Dale fan site. http://www.indyranger.com/disneyonrangers.html

Some Ranger “facts”—some from Stones, some attributed to him—on the history and development of the Rangers. 

From Stones

“The Rescuers was one of the first shows pitched after the success of ‘Ducktales’. Jeffrey Katzenberg nixed the idea because they had plans for a sequel (which became “Rescuers Down Under”).  When we developed ‘Metro Mice’ (which eventually became ‘Chip ‘n Dale’s Rescue Rangers’) he said to ‘make this your ‘Rescuers’’.

“In many ways the Rescuers would be a better canvas for a series.  Certainly there would be the chance of a repeating cast of international agents that would work with Bernard and Bianca.  But at the center, I think the chipmunks were more appealing to kids than B & B since they felt like such middle-aged characters."

Attributed to Stones

Interview by Joseph Luna - When I interviewed Tad Stones (for a UCLA class), he showed me his original character designs, before Chip and Dale were integrated into the series. Chip's role was filled by a bland-looking Indiana Jones-type mouse; Gadget's role was filled by a chameleon (!) with glasses (I think; it was a while ago); I don't remember the others, except Monty was basically the same. 

When the chipmunks were introduced, and further revisions adopted, the final picture emerged.  But Stones’ designs were a little more "edged".  Gadget, especially, was less cute and more... attractive, I guess, is the right word.