From Muppet Wiki

A substantial number of the Muppet characters are most commonly seen unclothed, although ironically, the always fully clothed Janice is the only outspoken proponent of nudity. Outside of costume roles and recurring skits, the following characters most often appear in the felt or foamy altogether, save for the occasional accessories such as hats, ties, or bows. The majority of these revealing characters are animals or monsters, thus curtailing concerns over flagrant self-exhibitionism.

• Kermit the Frog (wears only a collar)
• Rowlf
• Fozzie Bear (wears hat and tie)
• Robin the Frog
• Sam the Eagle (He realises this fact while delivering an editorial denouncing nudity. "Could someone please hand me a robe?")
• Bobo the Bear (security guard hat occasionally)
• Thog
• Doglion
• Snowths
• Basil
• Sprocket
• Bear
• Tutter
• The Cat in the Hat (wears a bow tie and hat)
• Tingo
• Timmy Monster
• Fletcher Bird
• Gloat
• Boppity
• Beautiful Day Monster
• Miss Kitty
• Wilkins and Wontkins
• The LaChoy Dragon (chef's hat)
• Grover
• Cookie Monster
• Oscar the Grouch
• Big Bird
• Elmo
• Herry Monster (although in illustrations, he's invariably clad in pants with pink stripes).
• Mr. Snuffleupagus
• Alice Snuffleupagus (bow)
• Telly Monster
• Zoe (barrettes and jewelry at first, now sometimes wears a tutu)
• Rosita
• Many Sesame Street Monsters
• Baby Bear
• Papa Bear (hat and glasses)
• Rizzo the Rat (in early appearances only, now has bowed to pressure from the establishment and is fully clothed)
• Camilla and other Chickens
• Rugby Tiger
• Candle
• Sprocket
• Boober Fraggle (hat and scarf)
• Philo and Gunge
• Doozers (helmets, hats, toolbelts)

In the teleplay for All Work and All Play, an episode of Fraggle Rock, writer Jerry Juhl describes Doozer wardrobe on the morning of Cotterpin's helmet ceremony: "The female Doozers all wear little flowered hats and the men are in bowlers. Otherwise they are nude. This, apparently, is formal attire for state and religious occasions."