How about this:
She knows how to use the microwave. Or this:
She has trouble just boiling water.
Or, more seriously, this:
She doesn't exactly cook like a gourmet chef. Hmmm... I'm not sure people would usually use 'cook' as a verb to describe what you want to describe. Instead, I think 'cook' would tend to be used as a noun, accompanied by whatever adjective is a suitable description of her prowess as a cook.
She's a fabulous cook.
She's a talented cook.
She's a good cook.
She's a so-so cook.
She's not a particularly good cook.
She's an awful cook.
She's the world's worst cook.