Rampage totally said he can clean himself, there is no evidence to the contrary what do you mean. Luckily, First Aid's teasing makes sense in either context!

"I can," he huffs. "I just didn't feel like it. Cut that out."

First Aid's nuzzle leaves him feeling distinctly flustered, and he can't actually huddle any closer to the ground. He twitches his mandible and snaps Hus claws restlessly instead

It's actually a relief when he goes back to cleaning. First Aid doesn't need him to fluff up much, he's already a ruffled crab!

"It's good for resting-" There's something comforting about bundling up in a hole as a ball of crab-monster. "And more stable on rough terrain. Not sure I have a preference."

They're both good modes. The only one that gets a bit ignored is his tank mode.