I just made a huge batch of buttercream and accidently made it a thin consistency. I usually use medium consistency under fondant covered cakes. Will thin buttercream just smoosh all over the place underneath the weight of fondant? Adding more powdered sugar will make the buttercream way too sweet but i'd hate to throw this batch out and start new. Any advice? Has anyone done this with no issues?
Thanks!

Just ice your cake normally and put it in the freezer for 15 minutes before putting fondant on it, so you are applying the fondant to a hard chilled surface. Easy-peasy, no worries.

if i chill the buttercream, when it starts to defrost, will it make the fondant look bumpy or start seeping from underneath the fondant?

Nope. As long as your buttercream was nice and smooth to begin with, it will be just fine. Smooth your fondant quickly and let it set for a little bit just in case the fondant gets a little bit sticky, which happens from time to time. I never put fondant on room temp buttercream - it is infinitely easier to do it on chilled buttercream.