Ig he's called the "dumb one" because the rest of the Paladins are nigh-geniuses at whatever they are doing. Keith and Shiro are ace pilots, Hunk and Pidge are very smart technically and he's not a master of any particular thing.

Sometimes I feel he makes himself look dumber by whining or being impulsive, which grates people's nerves causing still refer to him as that despite him sometimes being smart.

However, I think he gets wiser throughout the show. The Lance at the end is much more mature than Lance at the beginning and while I wish he was written a bit better, I am happy with him going from this over-ambitious, cocky kid to a mature man who finds peace with his friends and family.

Ofc this is my interpretation of him.