After binge watching both seasons over the course of three days (Never do this), I can honestly say that this is among the best children's shows currently shown.
Unlike most kid's fare around these days it doesn't pander to random humor and trying to be "cool". It instead uses plausible situations and character development to drive a narrative forward, the way kid's shows used to be until the late 2000's. It carries on pretty much exactly where the first film leaves off and portrays Hiccup's difficulties in assimilating dragons into Berkian life and facing off against several different villains. Of these three, Dagur the deranged is the best written, and even I find myself smiling at some of his lines and mannerisms.
The score to the show is surprisingly well done, with John Ceazarone taking Powell's themes and implementing them quite well in each episode. The animation is certainly not on par with the film's themselves, and as a result Hiccup and Toothless can look a bit strange at times, but this is understandable and shouldn't impede the viewing.
The writing and plots are mostly predictable for the aged and educated viewer, but still has a surprise every now and then that redeems it. The jokes, while occasionally repetitive and catering to a young audience, still keep the show interesting, and there is occasionally a particularly good one that will stay in your head a while.
If you loved the first movie and now the second I would advise seeing it. You will certainly not be disappointed and it will give the fix of dragon you'll need to hold you over until the third film.