Bowser doesn't need to. He does really well against Little Mac since Fortress puts him in a unique position to punish everything Little Mac does. I can't remember the last time I lost to a Little Mac using Bowser.
Mewtwo...I am still figuring that matchup out, but Confusion is my #1 tool there so far.
I feel like you haven't played Tipster's Little Mac in a while from what you're saying here. Mac's fsmash (especially charged, and Mac has no reason not to charge against Bowser, that's a 50:50 in his favor) pushes Bowser so far away on shield that fortress isn't an option, and Mac can go into jabs or ftilt so fast that the window to punish is surprisingly small. Would love to see some replays of you beating Tipster in that matchup to see what you do differently...