Yeah, more people might play it if it's a WebGL build simply because it's less work for the player. As LazarusCraven said, people will still play downloadable-only games. If you only have a downloadable version, consider the following to get as many people playing as possible:
- Keep the download as small as possible
- Include screenshots
- Include a video
- Include instructions
- Have something written to share your game hook or premise without spoiling too much (if it's the type of game to suffer from spoilage)
All of these things apply to WebGL as well, to maximize players. It's a game jam, so people are forgiving of poor-ish optimization as long as the game is playable, and no one really expects professional copywriting. But putting in an effort will generally get rewarded with people giving your game a chance since that is usually visible from the person's page. Having a fun (and fun-looking) game helps, too. Good luck!
Additional Note:
WebGL has limitations. If you're not sure what those limitations are, especially for an unofficial solution to creating a WebGL build for Unreal, then it may be better to not even worry about it and just focus on a good downloadable build - otherwise, you might end up spending time on visuals or other gameplay elements that aren't supported in WebGL and will be disappointed.