Is it normal to let someone know that they smell bad?

Yes but be nice about it.

Don't be like you smell like shit.

I guess it would depend on how well you know them.

I wish MORE people would inform people they stink. I've always been afraid to do it, but if I stank, I would like it if someone would discreetly and tactfully inform. Start the trend!!!

Don't bother telling them, they know they stink , they just don't care. I guess it would be kind of rude to tell them.

If you care about someone you would let them know.. I won't lie I'd be a little upset and embarrassed but in the end I'd rather someone tell me who's close and care about me than some strangers talking about me or saying " hey you over there you smell like shit!!!!"... Lol

Give them a subtle hint like "Holy shit, it smells like your dog farted" especially if they don't have a dog.
or "It's hard to believe that you can smell the city landfill from here"

...in my opinion, it depends on the sitation...I once knew of a waitress who smelled bad, and another waitress politely informed them of the situation...I sometimes will tell someone (as privately and kindly as possible) that their breath may offend someone who is in church ministry or at work, if they generally encounter people a lot.

if you and them are friends yes

I'd get angry.

You smell like a dumpster... Thank you for telling me