If I'm walking down the street and I see everyone wearing the same half dozen or so outfits, it would look very weird to me. Put them in multiple different outfits and they will start to look normal. If they add more zombies, they need to balance them and make them unique. Adding new zombies but having them act like existing zombies doesn't provide any benefit other than a different look and you can only make a very limited number of those that way. I would rather see a horde coming at me where exact duplicates are uncommon than to have a few more types of zombies that will still look repetitive as you'll have only a few new ones compared to the possibility of hundreds of different looks if you used skins on existing zombies. I'm not saying they can't also add new zombies, but you just won't get much variety by doing it only that way. With skins, your hordes can look far more varied and less repetitive. Your quest zombies will feel less identical and more like they were real people turned into zombies instead of a half dozen people turned into thousands of clones who were then turned into zombies. Ideally, you'd have different clothing that is similar enough in style so you can still identify what zombie you're fighting for strategy purposes, but still different so they seem like people in just their regular clothing. But the easier option is just to tint the existing clothing with different colors so they at least look different enough to not feel as repetitive. For the amount of effort spent, you can get far more out of using skins than making additional unique zombies. Even if they added 10 more unique zombies, it will still feel repetitive because you still don't have that many and when you have a bunch coming after you, there will be all kinds of duplication. If they added multiple skins to each zombie, then you can quickly end up in the hundreds of possible looks with far less work. If they could get a bit more modular and have a few hair styles, a few skin tones, a few colors of clothes and perhaps even a few clothing styles for each zombie, then you would have so much variety that you would hardly see exact duplicates. You just can't get that from only adding more zombies.
I'm fine with a grid UI. Those work very well (at least if they are well-designed). I just hope the style can be adjusted or easily changed, preferably without needing a mod. The style feels more sci-fi or at least crisply modern than what you'd expect for a post-apocalyptic world. The interface should have a DIY, dingy/rusty, Mad Max type feel to it, imo. That UI (mainly the stat bars, but much of it is also too clean and modern for this game) is just not a good fit for this game. Please consider changing the style to match the game rather than putting some "pretty" and modern looking UI on the game. The UI should fit the game. A modern and clean UI like that is fine for other games, but really looks off in this one. You can still have the grid and have a look that fits the game as well.