It's not as popular as when the game first released for sure, but they're still pretty common both in Turf and Ranked. As to why it's not being used as much as before, well that's probably because we've all realized it's not actually that great and there's literally only one reason to use it competitively speaking.
Firstly the main weapon itself is simply outclassed by the Splash and N-Zap. All three have the same fire rate and need 4 hits to kill, however the Jr.'s accuracy is much worse than both, and with less range than the N-Zap. The Sub being Splat Bombs are very good, but not exclusive to the Jr., and even between run n gun type shooters there's the N-Zap again. That leaves us with it's Bubbler special being the main reason to use it. Yeah other weapons have Bubbler too, but only a few, none of which being run n gun types, and none of whom can fill the special meter as fast, so that gives the Jr. it's niche.
Perhaps after the Tri-Slosher, Zink Mini Splatling and Custom Hydra Splatling release we'll start seeing much less Jr.'s, but perhaps not as it's still the only run and gun type weapon with it. Only if they give something like a Splattershot the Bubbler would the Jr. truely fade into oblivion me thinks.