I originally wanted questions like these to be submitted on the Answers Wiki instead (and even mentioned as such in the Forum Policies), but I suppose I don't really mind if people ask them here as well. More opportunity for conversation than getting a straight-up answer.
But even still, here's the matching question someone asked about this topic.
My personal opinion: I thought in the beginning it had a good chance of being canon, but as the show progressed it lost that chance, and now I think it's just its own thing. The characters are way too OOC (especially Po), and tons of small story things just don't match up. For example, Shifu arriving at the Jade Palace because his father Shirong had left him there as a young kid, when in Kung Fu Panda Holiday he says that he purposely left his home. "Oh but he could've been lying, or maybe he didn't remember," you might say. Nah, I think the show writers just missed that. :P
There's lots more small examples like that that confirm to me the show is its own thing. Perhaps a couple episodes or events from it would be okay for the film canon, but most of it was just unnecessary and inaccurate filler.
(Just as an extra plug-in: someone asked about the show on the wiki's Tumblr, and I answered it. It has some more of my input on the show in general.)