Look at usage figures for the rail network over the last thirty years. Before COVID threw a spanner in the works, the numbers were going through the roof, and the introduction of new rolling stock often attracts new customers. The number of people using the Uckfield line, where ten-coach 171s now operate at peak periods, is a case in point. The Thumpers were, quite frankly, absolutely rancid, and however nice the lighting was in your opinion, they were driving customers away.
OK, there's a million and one variables involved, but there’s more than enough packed out IETs and Azumas out there to suggest the lighting isn't putting people off at all. It won't have even remotely crossed the minds of 99.999999% of customers (ditto underfloor engines).
I'm no great fan of the class 385s, which have fairly grim seats and poor window alignment, but since introduction, ScotRail have had absolutely no problem filling eight-car trains where previously six-car trains ran. I think the onus is on you to prove the lighting is putting people off. Clue: it isn't.
If you ask people why they don't use trains, the answers will be dominated by "they're too expensive", "they're not reliable enough" and "they're not convenient for me". Nowhere will you see "the lights are too bright".