Now that you clarified in a PM, having to mulligan is not a big problem, but it is a problem. Almost as bad as giving your opponent an extra card or two is giving them a look at whatever was just in your hand. For example, if you Mulligan against a Night March player and they see a Karen, they can adjust their playstyle to try and be ready for her. No, not a great example, but the simplest one to spring to mind. XP
If you're going first, you might be able to slam your opponent with N, mitigating the advantage, but there are decks that want a different first turn Supporter, or just won't be able to get the N. All in all, one or two mulligans don't matter much. Oh, and you'll need to consider the opponent and the deck as well; some decks and some players need the help more than others, or some can make better use of the help more than others.