I don't think there's ever any harm in asking either (the worse that can happen is the answer is no) but I'm a bit surprised as in my many years of renting (and still renting!) its never occured to me that I could request improvements from the LL, I've just always thought of the properties I've lived in as 'as is' but I can definately see that an equivalent barter (eg rent increase) could do the trick if the improvement is something you feel you really need and LL might benefit from.
Funnily enough as a recent newbie LL I got the first request from the tenant the other day that made me go huh
He asked for a front porch to be built, the answer was no, but no harm came from asking.