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I think that, to understand this question, we must understand why pokemon evolve.
Obviously, it's so that pokemon can be restricted to certain parts of the game. Charizard can't be obtained until level 36 because a level 5 charizard would be devastating to the entire early Kanto region. But that doesn't really make sense - why not just make all pokemon around the same power level, and eliminate evolution entirely? Well, keep in mind that some pokemon don't evolve. This forces you to give up on your Raticate and Farfetch'd as you find more powerful replacements, which in turn causes your experience of the game to be more diverse. So, really, the purpose of evolution is to allow mons you got in the early game, like Pidgey, to still have some relevance against the Elite Four.
So, a good evolution does exactly that: Make a pokemon more relevant in the late game.
On the other hand, I think you might be talking about aesthetic design in particular. This is, naturally, something no two people will ever agree upon - I mean, there are people out there who like Sigilyph, and I actually like Klefki.
In this respect, I'd say a pokemon's evolution can be judged two ways. Firstly, is the new evolution respectful to the original design (i.e. Charizard's tail flame?) Secondly, does it add enough to the design to justify its own existence (i.e. Charizard's wings?)
IMO, a great example of both of these is Mega Kangaskhan. It is, obviously, an extension of the original design, but it also adds to it - in a way that lends new meaning to the original. In contrast, Mega Aerodactyl just looks like the same thing with spikes added.
