Does using targets/dummies improve skills?

These dummies are just for effect. The Dragonborn cannot gain skillpoints from attacking training dummies, as they are not actually dealing damage to anything (which is the way to improve these skills).

Archery targets, however, are useful if you want practice lining up your shots, however, and are especially useful if you wait around until NPCs practice on them, as you can harvest the NPC's arrows from the target.

If you're bold enough to pickpocket more powerful arrows onto them, harvesting the infinite arrows from their targets is the best way to get a large supply of the most valuable arrows in the game.

NO, but.... it could it you wanted it to.

As Raven Dreamer said, no in the default game, however, if you're playing on almost any system that supports mods, and if you're interested in such things, the mod Practice Dummies by Arthmoor:

Practice Dummies allows you to train your 1H and 2H combat skills, archery, and magic skills with frost, fire, and shock spells. You can do this up until skill level 30, after which you will gain no more insight from attacking the dummies. This is accomplished through the use of a simple script attached to combat dummy objects which are already present in the game but never used. Archery targets required the creation of a second object to replace the normal static target objects.

This mod is oddly not available on Nexus but is on BethesdaNet by Arthmoor for PC and XBox One.

Arthmoor does a great job with mods and he's very prolific so definitely check out his stuff.