I don’t think it’s a big deal, in reality when these firms go public their valuations will be in line with each other. Right now circumstances are dictating things, N26 and Revolut were very lucky to get through in time.
Monzo surviving is more important than a paper loss on valuation, by the time they go public little will have changed other than slight dillution. It’s a 5% stake at a £1.5bn valuation. Not the end of the world.