Quotes:
" Organic mulches, such as chipped bark, well-rotted manure and garden compost, break down slowly and improve the structure of the soil, opening up heavy clay and boosting the fertility and moisture-retention of sandy ones"
and
"Chipped bark: Nothing sets off plants like a mulch of bark - the whole border looks instantly tidier. Choose the grade to match the scale of your plants - big chunks for shrubberies, small chips for herbaceous borders. Bark is a good weed suppresser, but it is expensive and low in nutrients. Available from garden centres; if you want to buy in bulk, it's cheaper to order direct from a local tree surgeon - just ensure the bark is well rotted"
Source:
https://www.theguardian.com/lifeandstyle/2008/mar/01/gardens1
So, if by 'improving' the soil you mean making its structure more plant-friendly, then you'll be well on your way to improving your aim (although, obviously, that's not going to happen until the bark has rotted down into the soil). However if you're seeking to add nutrients to the soil you'll need something else as well.
See also here:
https://www.rhs.org.uk/advice/profile?PID=865