I generally don't have an issue with a Pokemon's evolution being wildly different as long as I think it looks cool or good.Interesting. Though I think the bigger issue for me apart from the line's progression is the fact that I just don't like Blaziken's design at all to be honest. Which is why even though Dewott to Samurott seems like an incongruous leap as well, I'm not nearly as disappointed since I think both designs are quite good in a vacuum.
Dragonair -> Dragonite
Chespin -> Chesnaught
Magikarp/Feebas -> Gyarados/Milotic (although it's kinda the whole gimmick with these)
Shroomish -> Breloom
Dewott -> Samurott
All of these are Pokemon that I adore that don't really look like their earlier evolutions in a few ways, but I still like them because even if they look pretty different than their prevos, they still have good designs in my opinion. For example, I don't hate Greninja because it's different compared to Frogadier (even if I prefer Frogadier vastly). Same with Blaziken's line. And I certainly don't like Magmortar just because it looks similar enough to Magmar and an actual evolution of it!
Seriously I hate Magmortar. I can't believe that in Platinum, if you didn't pick Infernape, 1/3 of your Fire type options was Magmortar lmao