This is the age-old question when it comes to modern video games, and the answer remains the same with Fire Emblem Echoes: it all depends on how much you favor this game specifically and if you are willing to spend some of your hard earn money. However, generally speaking, this is one game where the DLC isn't really worth the cost. Fire Emblem Echos doesn't have the most difficult Lunatic Mode or any otherwise difficult content to it, so DLC for overclassing your fighting units isn't at all necessary.
The DLC that actually adds some of the more interesting content is a much better offer than some, but each of which are still quite expensive for what little it provides. Even if you're not interested in paying for some Downloadable Content, go ahead and download the Mila's Bounty Items... They're free!