You can use it, sure. Lekker is more broad than what you've heard. Indeed it's used for edible things, but it can basically be used for everything enjoyable or comfortable, e.g. someone attractive (lekker ding), nice weather (lekker weer), I'm sitting comfortably (ik zit lekker). And also sleep well (slaap lekker).
May be you heard that from German? The German "lecker" is only used for "yummy, tasty", so the Dutch "lekker" is really cute to German speakers, it is like wishing someone a yummie sleep or saying the weather is yummie. :o) Thus, "lekker" is one of my favourite words in Dutch! :o)
I feel like translating welterusten with good night is wrong, not to mention confusing.. Welterusten means sleep well, regardless of the moment of day. So I could say welterusten to a child who goes for their afternoon nap for instance. Similarly I can say good night (goede nacht) when I leave late at night to someone who isn't going to sleep (they might have a night shift for instance)...