Being better is a relative thing.
Saying "The Lions will be better!" is moot because that is only comparing the Lions to themselves.
The Lions will improve after the Draft, as every team will. Now, will the Lions only improve by 10% while their opponents improve 15+%? So, while the Lions improved, their opponents improved even more and therefore the Lions likely win fewer games than last year.
On the other hand, maybe the Lions improve by 50% while their opponents only improve by 5%. Then the Lions likely win more games than they did last year.
The baseline is also different for each team. Say a team has 50% better talent than the Lions to start with can afford a 5% improvement because the Lions would need to improve by over 55%...which will not happen unless they score an All-Pro with every single pick, which has never happened in NFL history.
All of it is relative, but every team improves after the Draft compared to their situation before the Draft. Adding young talent to compete and replace less talented players results in an improved team...but it is relative to the improvement other teams, combined with where each team starts out, that matters more.
So while the Lions will undoubtedly improve after the Draft...how much better will they be compared to how much better their opponents will be is what must be analyzed.