The border with Slovakia after the Vienna decision was very good in the sense that it reflected the ethnic boundaries pretty well (not perfectly). But its hard to keep as Czechslovakia was considered a winner (though the role Slovakia played could be worth investigating).
Actually post Vienna border was not really reflecting ethnic borders.
According to 1930 Czechoslovak census some 549 376 Hungarians and 431 545 Czechoslovaks lived on territories granted to Hungary.
Post Little war eastern border added majority Slovak and Ruthenian territories with almost no single Hungarian living there.
Well as to investigate role of Slovakia - Tiso was hanged for treason, his part on aggressive war against Poland and USSR and other crimes.
Also Slovaks had % more volunteers in Czechoslovak army abroad then was their % on Czechoslovak population (from some 50% in France to around 20% for most part of war to over 80% in 1944) and actually Slovak politician Osusky was man who kickstarted Czechoslovak Government in Exile.
Also there was this little thing called Slovak National Uprising, one of largest Uprisings against Germans in Europe which despite being defeated lasted 2 months.
Another thing, Munich was proclaimed null and void by Allies. Vienna award was consequence of Munich so end up same way.
And I am not saying border post WWII couldn’t be more just for Hungarians.
Problem in my opinion post WWI was Hungarians feeling lost of whole Slovakia was not just and basically they were not willing to accept loss of Upper land as a whole, not just ethnically Hungarian territories.
It would be interesting though if Poland and Hungary have seen possibilities of things going wrong for them after 1938 and offered Czechoslovakia support for straightening borders along ethnic lines based on plebiscite overviewed by international observers.
However this was not possible as Hungary jumped on German wagon which they saw as only opportunity to clear their territorial claims to basically all of their neighbors.
And to be honest I am not sure how would be such offer viewed in Prague. Benes was asked for years by Czechoslovak Generals to straighten relations with Poland. Did he managed to do something about it?
For Slovakia borders post Vienna were economically survive able - Slovakia was able to feed itself basically to summer 1944 and even export. More just border would be even easier. Especially if deal was accompanied with right to pass for trains without hold ups.