Sorry i can't agree with you I don't think the Villarreal team is stronger than Barcelona. This was not an important match for Barcelona. Because second place in the points table was guaranteed for Barcelona. Barcelona also lost to Cadiz this season. Now would you say the Cadiz team is stronger than Barcelona? This is not reasonable. Moreover, even if Villarreal won this match, they could not dominate.
I think the size of the strong or weak cannot be determined from one or two matches. A strong team is a team capable of winning titles because he is able to perform better despite their few losing games. So one game cannot be a measure of strength or weakness. So far I think Barcelona have better quality and finishes than Villareal and cadiz so I think Barcelona are the better of those two teams.
You are right. Barcelona has lost a number of matches against weaker teams this season. This does not mean that the Barcelona team is too weak. If Barcelona had not secured second place in the points table, they would have tried their best to win against Villarreal. Barcelona is a strong enough team now. Next season they will try to organize themselves more. They will try their best to win the title next season.