Rather than just say you should know can you explain why muesli, apple and milk are bad? Because if I googled all of them things I know for sure no one would be saying it's bad to have muesli, apple or milk.
What does your diet consist of? You're a big guy no doubt most of it just the way you're built but still.
Refined/process foods, and heavily milled foods are poor quality. Muesli is generally that. However, if you can secure some unmilled un processed cereals and make your own - then of course we're dealing with a different case.
Apples are fine.
I'm not a fan of mass produced dairy - for the same reasons, refined/processed and nutritionally poor. However, if you don't care about that, then you should keep at it. At least go for full fat - or unhomoginsed if you want some decent flavour.
If you DO want good quality milk, fresh unpasturised milk is the best - as you get all the nutrition - but then you have to be careful where you get it from.
Ultimately it's up to you. I don't eat crap - I'm boring, I'm a health/fitness/food snob - and I like a certain quality of food and can afford to be picky about what I eat.
I've always been against heavily refined/processed foods - and it's hard in this day and age to get away from it unfortunately.
Cereals in general are poor - especially the mass produced stuff (then again what stuff isn't?).
However, I do understand that for most people, they don't really care, and that it's too expensive to live this way - and for that I do not hold any contempt - because it IS expensive. This is my bone of contention, this good quality food shouldn't be expensive, and good fresh produce SHOULD be available to all at a decent price. However, the cost of mass production and poor quality refined foods haver been so low and is a hangover from the wars - we were living off refined/processed foods as a result of the world wars and it's effect it had on farming, and food. As such we've never really recovered from that behaviour or addiction and it is now the norm. It's just part and parcel of the British culture to eat food from a can or a packet and think it's "good" whereas it really isn't.
A lot of the other parts of the world, taking the Med for example, live of fresh produce and as such have a far healthier and better diet - and this is proven in terms of longevity as well as heart diseases as well as other gastro-based diseases.
Anyway, this is going off topic, I'm not here to share fact with you or tell you what you should and should not do - I'm just giving you basic truths about modern food in this country.
In principle, there's nothing wrong really as it's for you to choose - and for no one else to judge - we can just provide you with the information. You have to do what you can afford and what works for you. In this day an age we're stuck with what we've got unless you live in "dreamland" of nutrition and food. We also have to let our hair down and enjoy what we eat irrespective of where it's come from / what it is - I'm not perfect either, but I make advised choices.
The choice is now yours.
I personally like to try and follow as healthy a diet/lifestyle as possible - not just eat for the sake of filling some macros - it's not just about going to the gym and stuffing my face, but making conscious decisions as to what I am eating and promoting health.