No it's not foolish at all...I used to be one of those people who thought so, but now I can't get enough clothes for my little furbutts...
I have 6 of them, and they are all more well-dressed than their mommy...lol. There's nothing wrong with it, and it does keep them warm. It has never bothered mine to wear clothes, not a bit. In fact, if I have one naked for a little while and I pull out some clothes, they run over, ready to get dressed! All it is is a fashion statement, just like you dressing your child in something. Your relatives wouldn't make fun of that, would they? Just ignore them...you'll always have people who have different views as you (at first my mom made jokes about my dog's clothes, then she saw how cute they were and started buying them for hers too).