If I were a player in this game I actually would be worried. The DM doesn't seem to have a clue how to make a balanced encounter and after this is likely to just look up random Monster Manual entries of a CR six levels higher than the party or throw so many weak ones at the party that the action economy makes it impossible to survive.
You comment does assume a bit of context. They might be a new DM just learning how to do stuff, in which case it's perfect fine to not be perfect every session.
Heck, it's perfectly fine to not be perfect even for experienced DMs. The most important thing is that whatever happens both the players and the DM learned something.
Give 'em hobgoblins; if I recall correctly, those guys have like 18 ac and teamwork abilities. They're also really low CR; my brother's party faced a group of them and could barely hit them. Cheap for sure, but a guaranteed difficult encounter.