[Matrixbeast]

I feel Mega evolutions are best when they look like an advancement of the existing character, not just looking like they hijacked someone's personal interpretation of the Pokemon off DA and called it their own.
This is why I feel Pokemon like Mega Manectric, Mega Rayquaza, and Mega Scizor fail. They feel rather cheap, putting a giant halt to their general design in favor of something different entirely, usually too over the top.

The big issue is that it's very much a case by case situation, with some Mega evos looking gawd awful despite following their general design aesthetics, while some looking great despite departing from their general look.

I think the only thing I can think of for a "Good" Mega evo guideline is to not focus on one aspect of the Pokemon and go crazy with it (Like Alakazam's mustache or Manetric's spiked fur).