Why would a band call themselves Supertramp? Why should they demean themselves?

how do you know they were referring to themselves?

maybe they named the band after a groupie

The Logical Song took on a new meaning to me when I had children and now grandchildren because it really hurt to see them go to their first day of school and watch their childhood taken away from them forever.

There was once a point in time when the word tramp had no sexual connotations at all, pervert.

Adding the superlative reverses the connotation. If you're going to be a tramp (ie itinerant, bum, hobo etc), then be the best damn tramp there is. That makes a positive statement, not a demeaning one.

I'm assuming that you're an American. The word "tramp" can have multiple meanings, but only Americans use it to refer to women with loose morals. In the rest of the world, used as a noun, it refers to a person who "tramps" about (walks), i.e.: a "hobo." 

Pretenders, The Zombies, Humble Pie, Queen, Meat Loaf, Rolling Stones, Grateful Dead, Black Sabbath, Crazy Horse (Neil Young's back up band), Sex Pistols, The Animals, Steppenwolf, Los Lobos (Spanish for wolf), Dire Straits,and The Kinks are not flattering names either. Self deprecation is common among artists. They do not want to take themselves too seriously because otherwise they would be too easily devastated by the critics.

They took the name from the book "The Autobiography of a Super-Tramp" by W.H. Davies.

Names are to be Remembered. Not to make a social statement.

With so many bands being just average or less, it is the Name that draws.

Have a "good" name and it does not matter much how well the "music" is.

I suppose the Sex Pistols meets with your approval?