Serbia: mostly neutral opinion. The Balts are rarely mentioned, but of course people here are mostly rusophiles, so if Balts do something deemed rusophobic, people will trash talk them. Other than that there’s some rivalry in basketball between Serbia and Lithuania and that’s pretty much it.
I personally think of them (in ethnic sense) like distant cousins, more distant than other Slavic nations, but closer than Germanic or similar. But in general I don’t have any formed opinion on them.