I'll answer, but please don't take this as definitive - I'm not actually that 'regular' a Civilization VI player; I don't have the experience with the gameplay itself to know, instinctively, what strength such a bonus might have.
It doesn't necessarily feel 'too strong' outright - but it does overlap the Chichen Itza wonder a bit. Or, at least, generate an obvious 'boost' from coupling that civilization with building that wonder. I think the 'good thing' about using feature-based bonuses is that there's a level of randomness attached - assuming you use sensible FeatureStartBias settings, you are not setting them up for guaranteed boosts from the early game. As with many boosts of this kind, it's potential-to-overpower is accentuated early if coupled with an appropriate start - +1 can be an effective 100% (or more) increase versus 'the average'.
The other obvious place to look for comparison is the Brazilian civ - as they have an ability that revolves around the rainforest. Now, Brazil get some adjacency bonuses from rainforest tiles - which, to me, makes it an altogether more interesting bonus. It's one-step-removed from the tile itself, which makes it feel a bit more dynamic and not a flat increase irrespective. You know, it requires the player to do something strategic to unlock the bonus.
For that reason, I'd be tempted to say it is "+1 culture from rainforest tiles IF something else is also true". At that level, you can probably not worry about it being overpowered, which also means if you have other creative ideas, you can easily do something else as well. Perhaps it could grant +1 culture from a Unique Improvement that the civilization could build on rainforests? Straight away, that introduces the strategic element to unlock the bonus.
As a flat +1, I would be tempted to say "yes, but don't give more bonuses on that ability" and in that configuration, it just feels a bit, well, boring. In general, I always feel that core-yield-bonus-on-basic-terrain is too straightforward. Unless it thematically fits and is given a cool name with a bit of backstory that makes the player say "ah, that makes sense. Cool!"
