When I say "the waifu sticker" I don't literally mean exactly stickers with pretty anime girls on them. I don't really find anything wrong with the average peeker or anything, and honestly half the time the waifu sticker isn't even a sticker, it's a t-shirt or some other memorabilia.
What I mean is, in broad strokes, a piece of merchandise that is generally easy to produce (such as a t-shirt or a sticker) that usually features an anime girl (generally from a popular show like Darling in the Franxx, Nagatoro, SAO, Dress-up Darling, etc.) that is pretty traditionally attractive (she'll have big breasts and a big ass, but not too big), and often times has some hint of very basic eroticism (a bunny suit, a seductive pose, heart eyes, a pantyshot, etc.). I'm also going to lump in the "one word" stickers in here, where the whole point of the sticker is generally just one "anime" word. Notably, I'm not saying a Japanese word, because "waifu" and "senpai" are about as often to pop up as "lewd" and "weeaboo", it's more general western anime consuming culture than anything from the shows themselves.
General western anime consuming culture is honestly a great way to start poking at my normal frustrations with it. Anime is popular now, to an extent. If you go to a "pop culture" store anime is probably there, if you go online you'll find plenty of people who'll reference Jojo or post a Zero-Two reaction gif while being otherwise normal. It's normal to like Jojo, or Darling in the Franxx, or MHA, etc. It's pop culture, at least the shows that are popular, and every pop culture hit has its very own unlicensed shirt sold at a mall booth.
The Waifu Sticker is appealing because it helps you look like you're with the zeitgeist with very minimal effort, but in an edgy way. "Oh, you like anime? Well, me too, but my anime girl is in black and white and she's not wearing a skirt, isn't that so weird and far out?". It's signalling being in the in-group in a way that is digestible enough to be handled by the kind of person who'd search up "waifu" on redbubble instead of browsing through indie sticker makers, while also feeling digressive enough to settle down thoughts that you're just buying into something popular. Nonono, you're not buying a Bart Simpson shirt directly from the Universal Studios gift shop like everyone else, you're buying the unlicensed Bart shirt where he's smoking weed. Isn't that just so much cooler?
The other aspect that kinda gets to me is the sexualization, in the sense that a lot of it is very sexual, but in an incredibly bland way. If you as previously mentioned search for "waifu" on redbubble, assuming you're logged in and have the safesearch turned off, you'll be greeted by a variety of asses and tits and thighs. The worst you'll get is a bit of bondage here and there. It's incredibly toned down, stripped down, bare, and powerfully unerotic, primarily because of how it flattens a ton of characters into the "waifu" theme. Marin Kitagawa may be loud and proud, with a serious love of cosplay and a bit of an impulsive streak, but the important part is that she has relatively large breasts and a large ass. Nagatoro and her relationship with her senpai, how they slowly grow on each other and start pushing the other to do better and be better, isn't something that's allowed to manifest when you're trying to sell a $3 sticker based on the same handful of traits that all the other girls in your sticker shop have. Hell, half of the time they might as well not be a character, considering how they have a black bar covering their eyes with "waifu" covering slapped on the front. If you enjoy anime, even if you only enjoy a handful of very popular shows, trying to show that a character is your waifu by getting a sticker that explicitly covers up a big portion of their identity while flattening the rest of their existence into boobs and pantyshots seems like a genuinely terrible idea if you have any hint of genuine feelings.
But if your main goal isn't to have genuine feelings but instead to perform "liking anime in a mainstream way", then hey, whatever works!
I'm generally classifying this as rambling because complaining about this feels somewhat pointless. I'm getting mad over something that honestly kinda doesn't matter. The existence of someone with a Ryuuko KLK sticker of just her ass in panties in place of a personality doesn't mean I can't watch Kill La Kill and genuinely enjoy seeing the way it explores its themes and history, or the same with Dress up Darling, Mobile Suit Gundam, Lucky Star, anything else for that matter. I don't really interact with the kind of person this product is for, I have actively stayed away from most anime forums for a decade plus and counting. So I'm honestly confused why I care so much.