I personally find that a very odd perspective at this stage. He's on 10 league goals by Christmas, alongside £90m Lukaku and ahead of £60m Morata. He's done that as a supporting forward in a team that's really struggled to create chances - his 10 goals came from 24 shots, compared to e.g. Kane's 15 goals in 75 shots. He's scored arguably the goal of the season. And he's scored against City, Liverpool and Arsenal - the first two securing impressive draws, and the other a beautiful strike.
If you were arguing against people claiming he's world class, I'd understand the sentiment. That isn't the case though. No-one thinks he's world class. No one thinks he's one of the best players in the league. On the other hand, lots of people had suggested Everton wouldn't even want him. Lots more thought Everton made a mistake signing him.
The reality is that he's one of just two players to reach double figures in the league for Everton in the last 5 years, and he's done it by Christmas. He's currently on track for 20 league goals, which is a feat Lukaku only managed once - and as soon as he did he was sold for an astronomical fee. It's actually a feat Rooney only managed twice previously in his career. Across his entire career he's averaged 12 league goals a season, and it's pretty likely he'll go above that this year. If he gets over 15 league goals he'll be one of just two Everton players to do that in the last two decades.
To describe any of that as average seems a bit out of sync with reality, IMO.