To be honest most of these pictures of Meconopsis leaves look fairly normal to my eyes at least. The leaves are quite brittle and fragile and there can be dark patches on some of the leaves under wet, garden conditions, simple trick is just to remove any obviously damaged or older leaves and keep things tidy, mulching is good too and helps keep the plants in good health.
If you are talking more about botrytis inside a greenhouse or cold frame environment, that isn't unusual either, I'd recommend to minimise this issue, using flowers of sulphur which is quite inexpensive and dusting seed trays heavily with it and then afterwards fine alpine grit over the compost. Try to water in the morning rather than the evening too as this will prevent the emerging seedlings sitting overnight with a lot of moisture on their hairy little leaves. If you have the trays on capillary matting, you can water directly onto the matting rather than the seed trays which again might help a little and try to get your seedlings out of the greenhouse by around mid summer as I find that is when fungal problems seem to rear their ugly head and the young plants will grow much better, more healthily and more quickly outdoors at that time of year as they don't really need any more protection apart from maybe slugs which can be a problem when the plants are still quite small.