He played his way into Wimbledon. Marcus Willis earned the right to play in the tournament. He didn't just show up and pay to get in. He had to win 6 matches to qualify. That doesn't necessarily make him an elite player, but who knows, if he continues his efforts he could be one day.
Ranking doesn't especially matter; at any given moment, in any given match, anybody could possibly win. If Mr. Willis played well enough to get a chance, then he should certainly have had the opportunity to play out that chance at Wimbledon. Sport is meant to be an equalizer, not another source of floating cream.
Marcus WIllis worked hard to get to where he is and I think he deserves to play WImbledon. If he qualified and showed his skill by defeating the amount of players needed to move on to Wimbledon he should be allowed to play the game. However, I can't say for sure since I don't follow tennis closely.
Everyone would like to believe that average people like Marcus Willis can defeat great tennis players like Roger Federer and make it all the way to Wimbledon. However, this is simply not the case. When Willis had his first big win over Berankis, people instantly assumed that Willis was an average tennis player who could defy the norm and beat professional tennis players. Marcus Willis beat Berankis either because Berankis had a bad game or Willis had an unusually great game. Professional tennis players spend most of their lives dedicated to training. It is only fair and logical for these players, not average players like Willis, to make it to Wimbeldon.