Marth has been considered a relatively easy-to-learn character since Melee (though hard to master), and Lucina is easier Marth, so yes, she should be a very solid basic character. Of course, her lack of projectiles will make dealing with projectile users harder.
At a basic level, my normal advice for learning is to suggest people work their way up the CPUs until they can beat a Level 9 no sweat. That level of dodging and grabbing practice will easily beat a lot of low-level players, but in the process you can develop a lot of bad habits (like predictably dodging the way the computer wouldn't attack, or always grabbing after a shield).
Past that, the best way is to just keep playing. Play against more opponents, play against friends locally, play teams, play 1v1, just play a lot. Especially in friendly matches, try new things like going for more Counter hits, or starting attacks with Dancing Blade. Try not to become predictable (it happens at all levels of play, but the less predictable, the better). Practice trying to meteor-smash people with Down-Air, etc.
For people really intent on improving, it can help to save replays and watch for your own mistakes. Be aware not only of what the opponent was doing, but of what you were doing and why.
Also, it's easy to decry all use of projectiles as "spam", but there are several characters (Ness, (Dr.) Mario, Robin, etc) that have projectiles that are rather ineffective when spammed (at least against decent opponents), but can help cover approahces or combo into finishers at certain percents. Robin, especially, is wholly unable to spam spells more than a few times in a row (same for Smash attacks), and in that regard, can actually serve as a sort of training character to teach players to not rely on the same attack too many times.