"I'll be honest - I thought at times we weren't so great," Fletcher said after the five-try mauling.
"I thought our work-rate, our commitment and togetherness were good but we were at times a little bit shapeless as well."
Newcastle's tries admittedly owed more to woeful Wasps mistakes than inspired play, coming courtesy of their outstanding captain Matt Burke, Jamie Noon, Matt Thompson, Mark Mayerhofler and a first for the club for prop Joe McDonnell.
Loki Crichton converted three of those scores as well as kicking two penalties.
"I think we were pretty fortunate with all our tries. But you've got to finish them off," Fletcher added.
It was the effort up front which earned most praise from the Falcons boss, who said: "Our forwards to a man were very impressive. I thought Ben Woods was particularly outstanding, as he's been all season.
"We created a lot of turnovers, some unforced errors by them.
"But also some errors through us just going at them."
Despite establishing a healthy advantage shortly after half-time, Fletcher admitted he was concerned when Wasps eventually scored a try of their own through Mark McMillan, with last week's controversial late collapse at Bristol still fresh in the memory.
"We spoke about it all week about keeping a cool head, closing games out," he said.
"The memories of last week were coming back to haunt us a little bit."
As for whether his side, who are now 13 points clear of bottom side Worcester, are safe from relegation, Fletcher said: "Absolutely not. A good team's going to go down this year.
"We're out of nothing. Worcester and Northampton are going to win some games - they've both got to play us.
"We want to try and get in the middle of the table and kick on from there."
Fletcher did have some sympathy for a play-off chasing Wasps side who have seven players on Six Nations duty this weekend, compared to just three for his own side.