Cook together sometimes, especially the dishes your wife prepares for you that you value most
I endorse everything Rainbacon's answer says, and have a complementary suggestion: cook together sometimes.
It's pretty easy to underestimate how much variation there is in cooking ability and technique. When I was a kid, my friends and I cooked often when hanging out because if we wanted food beyond chips, that was the only way we were going to get it. It never occurred to me that that was unusual until college, when I met a lot of people who had never cooked anything more complicated that boxed macaroni and cheese (if that!).
It also never occurred to me to question the cooking techniques that I'd become accustomed to. We did what made sense to us, and worked well enough. Sometimes those techniques were really good, sometimes they were OK but idiosyncratic, and sometimes they were flat out bad.
I mention all of this to express that many people have internalized cooking techniques without necessarily having had enough cooking experience and feedback to develop the most effective approaches or account for varying tastes. And it can be difficult to identify which steps in the cooking process are causing specific outcomes in the resulting meals.
The end result of that can easily be formless, unhelpful criticisms. Your wife may be aware that you don't like her cooking, but unclear on which specific things she should do to improve it for you, or even how to go about meaningfully changing how she works in the kitchen. She also doesn't have access to your sensory experience of eating the food, so she can't isolate your feelings about it unless you tell her about them.
It is incredibly frustrating and demotivating to exclusively hear "this is bad, next time make it better" and then get no additional information or help. Even saying something like "this chicken is overcooked for my taste" doesn't necessarily help much, because it doesn't tell her what the correct amount of cooking would be like or help her assess when the chicken has reached that point on the stove or in the oven.
If you cook together sometimes, you can compare and demonstrate cooking techniques in real time with the exact dishes you'd like to see improved. If you show the process you follow when making a dish the way that you like it, you're giving constructive feedback on your preferences and also showing methods that you know produce the results you like. You might also notice practices that produce results you know you don't like, and can express that as well. Those are much better than a flat demand that things become abstractly "better".
As an adult I regularly cook with friends and family, and seeing how they cook food themselves helps me a lot to figure out how to prepare foods the way they like them.
Note 1: "I feel compelled to say something" and "I'm bad at giving compliments" are not impressive here
You feel what you feel, and it's not my place to tell you that you are definitely right or wrong about that. But it's obvious that "saying something" is not effective here, and so I wonder a bit about why you feel compelled to criticize "in order to ensure a better meal next time" when it seems like that outcome doesn't follow.
The nature of your comments and the manner in which you deliver them are very important, and indeed that is what interpersonal skills deal with here. As you are asking your wife to change how she does things to be more to your liking, it seems appropriate to at least consider changing how you go about things to be more to her liking. Practice giving compliments, if you think those are lacking, and practice criticizing less (or more gently), if you think you are too negative or harsh.
"You need to improve X, but I do not need to improve anything" is a difficult position to work from. If your attitude is that it is unfair or inappropriate for your wife to continue cooking food that you feel is subpar, it is an awkward stance to effectively claim that it's perfectly OK for you to discuss her cooking in a manner your wife dislikes simply because that's already how you happen to approach it.
So when you speak with your wife about her cooking, good interpersonal skills will involve changing the balance of complaints and compliments. However good or bad you feel you are at those things is immaterial to whether or not they are part of an approach that leads to the results you want.
Note 2: Your wife is helping you out
If she's cooking meals for you because you don't have time to do it yourself, her assistance is in saving you time, not in exactly replacing a meal you would have made yourself. Saving you the time is a favor she does for you, and in a case like that criticisms without anything positive to say can feel extra insulting. If you don't value the contribution she's making, or you legitimately have trouble eating the food, you don't have to eat it.
Cooking food my significant other likes is one of the joys in my life, and I send her off nearly every weekday with a lunchbox full of leftovers from food I've prepared for her. But if she started complaining about every dish I prepared and had little or nothing good to say about it, I would become irritated and my contributions to her lunch would quickly become something like some McDonald's coupons.
When you speak with your wife about her cooking, try to bear in mind all of the effort she is making on your behalf. I doubt she's intending to make food you don't want to eat, so consider emphasizing the work she's doing for your benefit more than the outcome that happens not to suit your tastes.
