It's all down to opinion, and since Game Freak don't exactly give you a list of base stats, the fact you don't see the ability change all the time and you don't see any type changes without going into a menu, appearances are the first thing you see - and in fact, I'd argue Mega Evolution is largely based upon impressing with dazzling imagery and appearances. That's of course one of the advantages of a visual medium; the fact that you can experience some really great artwork and design. In addition, it's basically a super mode - much like Super Sonic of Sonic the Hedgehog, the Super Sayains of Dragon Ball Z or the numerous powered up forms from too many mech anime to list, it's got to be something that just looks impressive and imposing, and something that fits the character - if your favourite Pokémon's 'super form' lacks that little something, you can be found feeling a little... underwhelmed, even if it does perform well in battle.
Luckily I haven't really felt this way about many Megas. Aside from the Latis I feel all of them either look impressive or improve upon the design and concept in some way, even if some - Mega "Flowerbutt" Venusaur - aren't quite as obvious as some others like Lopunny or Blastoise.
tl;dr - appearance is still important, especially to casual players, tho imo none of the designs are bad except mega latis