I'd say that opposite. For my case - I main Sheik. By copping out of difficult situations with 'specific tactics' (ie. Having a Pocket Fox to use his upsmash and upthrow upair against Puff), you aren't putting yourself to the challenge of LEARNING the matchup with your main (Sheik involves Bair and Fair zoning against Puff with needles and very stealthy grabs and throws). By relying on these specific tactics, you are halting potential growth that could be instead, invested in your main character.
Personally, I believe Sheik vs Puff isn't an impossible matchup and KK/Overtriforce show it's clearly possible to win against HungryBox.
Unless the matchup's almost completely one-sided, like Puff vs Peach or Peach vs ICs and you feel the absolute need to win, then it is not in your favor to opt to a secondary. From the first place, you should play Fox/Falco to avoid such situations if your main intentions are to win.
This has always existed as a truth for me when I started playing fighting games - Before I read and agreed with Umbreon's post. I come from a Street Fighter background - Where counterpicks are plentiful and commonly used. That still doesn't, for instance, stop Daigo Umehara from making Evo top 8 five times in a row with the most known, gimmick-less character of the game: Ryu. Some of the best players in the game have not opt to play secondaries and as a result, developed characters that propel far higher up than others. Again, making another reference from street fighter: Wao plays Oni solely in Street Fighter and destroys most of the top Japanese players in Topanga Cup.
Just want to say now before assumptions are put up: Just because most of the God 5 aren't playing sole characters does not mean that it isn't possible to succeed with other characters. You'll probably win the most with one main and one secondary that covers your main's stage disadvantages/matchups, but you'll limit the amount of growth possible for your main by investing that much needed time to your secondary. This is my wholesome opinion in regards to playing the game with a single main only.