No body commented on the brown tho [and the melan isn't even brown, so idk where that came from anyway?], the comments were that the sprite doesn't really look like a mega which I think is fair. It looks like its just an alt design for bunbori, instead of a powered up version as megas are meant to be.
Mega Absol, the Mewtwos, Gengar, Audino, and Lopunny look more like alt designs, but I don't really see too many complaints about that (with the exception of gengar)
I'll admit M. Bunbori is a little under-designed, but
On Bunbori, the mega has literally nothing new. Whereas on all of those megas, there is something new. Bunbori has things shifted around. That isn't a mega. As I said before, it looks more like a pre evolution.
Those megas, you listed, can't be changed. Their sprites can, but their designs can't be. Since most canon mega sprites seem to be getting remade anyways, however, I don't really think it matters what people think of those currently at this point in time anyways though.
Bunbori's design and sprite *can* be changed however, should the user feedback make an impact. Those mega pokemon you listed, at least they have new parts to them. Bunbori just had things swapped around. It got a longer tail, and some gem things on the tail, but that's it. Absol got wings and horns. Mewtwo got different bodies entirely. Gengar too. Audino got longer ears and those.... idek those string things. Loponny got longer ears as well and some more smaller stuff.
I really don't think it is a ton to ask for a new sprite, mega mew was changed extremely fast last year when the users showed their unhappiness towards it. Nobody from staff has even acknowledged our feedback on the mega here yet.
Just saying.