Allignments, thought and choices?

[dps]

My character's been pretty trusting and does a lot of things on a whim. Kind of flighty in nature and doesn't focus very well.

If this was already an established character trait of your bard, that makes the position your DM has taken here far worse IMO.

And how DM says I'm supposed to think as neutral good.

He has no business telling you how your character is supposed to think.

Should I from a roleplaying perspective be more proactively asking about these kinds of things? I'm kinda worried that if I spend to much time on that kind of thing it would cause a lull in the gameplay or seem too fake.

Do you want your character to become less trusting and naïve? If so, yeah, sure. If you want him to keep the same personality, no.

DM makes a good point that I should start to question things more and I should probably slow my character down a bit and think more about the consequences of my action. But I think that's a more gradual thing for an impulsive happy character to learn. Maybe flashback to this scenario and be more careful in the future.

Again, this is for your character to decide, not your DM. And even if you do want your character to become more thoughtful, alignment has nothing to do with it.