Chrom is so strong because he has high damage output, has long range because of his sword and a lot of his moves take priority. His only real downside is that his recovery can very easily be gimped by some characters. Although I would not call Chrom broken by any standards.

I just got into Elite Smash yesterday and quite a few people play Chrom, Inkling and K. Rool.

Nah, not anymore. Still strong and in high-tier but she is no longer the monster she once was - thankfully. Inkling, Chrom, Pikachu, Peach,Olimar, Yoshi, Marth, and a couple others I'm missing are considered top tier. All subject to change though as of launch. You picked a good one with DK, he's considered the best heavy by most of the pros apparently. I still rock with Ganon and learning to use Yoshi and DK. Maybe considering playing Inkling - not because of him/her being the top character - because I LOVE Splatoon 1 and 2! So, I'll keep it as an option. There's way too many characters to just pick up and main though, I love the variety!