A literary piece is always open to interpretations. But, in this case, I don’t think that Kaspar is using the terms ‘great victory’ and ‘famous victory’ in a sarcastic sense. Rather he has deep faith in the battle ending up in a great victory.
 
Kaspar is a common farmer. He also represents the conservative ideas of the old. If he is sarcastic in his statements, that means he is against the war. In that case, he would have taught his grandchildren against war, warned them about the destructive power of war. But in the poem we see him opposing the children. When Wilhelmine and Peterkin question about the validity of war, Kaspar stick to his ideals saying it a ‘famous victory’. He also says:

But things like that, you know, must be
At every famous victory.

If Kaspar was saying all these sarcastically, what would the children learn then? Would he not try to educate them with his ideals? Also, what message would the readers get then? The whole poem and its purpose would be in a mess then. That is why it can be ascertained that Kaspar is in no way sarcastic or ironic in his tone. But it comes from his faith in what others say.