Probably not at first, there’s obviously going to be a lot of leg work involved in getting the partnerships set up so there won’t be a comprehensive list of providers.
I’m not sure the marketplace will ever offer the very best deal for every product / service. I expect it will give some very good options & then most users will sign up via Monzo because they’ll make the evaluation / signup / on boarding / integration process so much better.
Plus Monzo have mentioned that they’re looking to include companies whose values align with theirs (& their users) and the company might have to be willing & able to share data via their APIs for integrations / a smoother on-boarding process so some might be left out deliberately.
I expect if you really want to shop around yourself, sometimes you’ll be able to find better deals. But obviously that’s time & hassle so you have to weigh that against the convenience…
Edited to add links