I agree that people's manners and sense of social decorum have sharply declined as of late, but I also think that there are huge regional variations in what is normal public behaviour and attire.
For example, there are a lot more people in the southern US that don't wear underwear and wear skimpy clothing in public, and a lot more people up here in Canada that wear big fuzzy hoodies, fleece pants, and snow suits in public.
People from the prairie provinces of Canada are usually shocked by the the over the top rudeness of most east coast Americans (NYC and Boston people in particular), whereas those Americans are being polite and friendly by their standards, and tend to get weirded out by Canadian's constant small talk with strangers in public.
I wear sweatpants a lot of the time for several reasons:
-I have meralgia parasthetica (seatbelt injury), and belted and or tight pants irritate it tremendously.
-jeans are in no way appropriate attire for physical activity or exercise.
-hipsters wear denim.
-sweats are available in many styles, cuts, and thicknesses, it's not 1985 anymore.
FWIW, I always put on clean clothes, and typically shower/wash immediately before going anywhere in public.
It is also a lot less common to wear a suit and tie here, in a typical week the only person I see wearing a suit is my pastor, the mayor of our city doesn't even wear a suit for most occasions.
Incidentally and anecdotally, if I do wear a suit I only get compliments from old men, because I look like them, not from women. However, if I wear clean colour-matched sweatpants and hoody and have new running shoes, I often have women approach me in public (this is also because I am married, women love a man that's already in a successful stable relationship).
On the few occasions I have had matching sweatsuits with my 9 year old son (he likes it), it really gets the ladies excited, it's hilarious but also extremely awkward/embarrassing for me, being an extreme introvert with social anxiety, but my son loves the attention.