As long as you table select, yes 50NL on pokerstars is soft. Other sites are softer.
Any level is soft if you only play against whales/fish.
But to just say that 50NL on PokerStars is soft implies that it's easy to earn good money there for a decent player.
You must put it in relative terms against other sites (and LIVE play) to determine if it's soft or hard.
And since you state yourself that other sites are softer it means PokerStars is harder.
If you beat 50NL on PokerStars for more than 5bb/100 after 100k-1million hands my hat is off to you. Playing 4 tables that is.
You are an elite player who needs no further advice on poker (at least not NL Holdem).