Sheik and Yoshi are literally Bowser's worst two MUs in the entire game. If you're losing to Bowsers, it's because you're not camping hard enough and/or or not pressuring Bowser in the air. Both characters also excel at gimping Bowser.
Contrary to what's been said prior to this post, Sheik and Yoshi abuse Bowser by attacking from BELOW him. Bad Bowsers will spam dair or Bowser Bomb as soon as they get above you. Just bait it and airdodge, then punish.
With Sheik literally all you need to do is charge needles and force Bowser to approach. Then when he does, do fthrow>fair, fair, fair, fair, Bouncing Fish offstage, and then read Bowser's recovery repeatedly with Bouncing Fish until he literally can't come back to the stage.
Congratulations, you've now hard countered Bowser.