Newspapers are fine, but if you want to kill brambles and grass,
cardboard is better. But more important than cardboard vs newspapers,
The secret to a good kill is to apply the mulch after growth has
resumed. That way you push down the shoot, and you have a long time
before the paper breaks down and lets perennials through.
There are plants that like degrading mulch, most notably tomatoes,
garlic, potatoes, squash. Degrading mulch tends to be acidic, but not
all the time, and not necessarily low nitrogen unless you use very
brown materials. Lettuce, for example, is sensitive to acidity, but it
will be very happy if planted directly through six month old leaves
mixed with some manure. In practice I always give wood ash to just
about anything i grow except potatoes (I have acid soil, and I prefer
wood chips, the most acid mulch of all, because I plant most
everything in seedling form).
Over time the pH of the degrading mulch climbs up to near neutral
values as it becomes soil. It will start to look like soil. There are
tricks that you can play. First, if you use leaves as mulch, they will
be 99% gone by next year, with a decent pH, so you can seed directly in
the resulting soil. If you use chunkier mulch, like wood chips, you
will have weed protection for two years or more, but you will have to
plant through the chips until they are gone. Wood chips start quite low
in pH but when they are done their soil is similar to that made of
other mulches, if possible with a stronger humus. If you use cardboard
covered with leaves or mulch, the cardboard is 99% gone the next year.
if your mulch is not quite done, and you want to seed directly there,
gently rake it to one side of the bed. Use that side for potatoes or
garlic, and the raked part for carrots and beets. The raked part will
have more weed seeds than if you had not raked it, but still less than
the soil underneath it.