Losing to Bowser can be a bit embarrassing, so yes, people will say that they aren't used to good Bowsers to john. Thing is, it's almost legit. I play Zelda, and when I beat someone at a smashfest with her, they remarked it was strange that I played her, no one does, and that they had no idea how to act. It's true that fewer people try to excel with the lower tier characters and consequently one of the best thing we low tier players have going for us is that few players are very experienced against these characters. It's sorta degrading to know that one aspect aiding in low and bottom tier success is lack of experience, but that's the way it is.
I recall one incident, before I devoted my training to Zelda only, when I used to switch between Zelda and Sheik in matches. I was playing a CF and performing well enough, though still getting beat. Halfway through my stocks, I switched to Sheik and I felt the player instantly felt more comfortable with a foe he was experience against. I didn't get a stock off of him with those last two stocks.