Lyon's appointment to the volunteer post will create a "problem in public perception," Garrott said Friday.
"It's not about Matt's qualifications," he said. "It's about the appearance of a conflict [of interest]."
The City Council must decide whether or not to approve the appointment. It will discuss Lyon's candidacy at its work session Tuesday.
Becker spokesman Art Raymond described Lyon as a "fabulously qualified" candidate whose nomination does not raise conflict-of-interest questions.
"The mayor's office would never put forward an appointment if we believed it was a conflict of interest," Raymond said. "We don't feel it's fair that a nominee's previous civic engagement should be a negative."
Councilman Kyle LaMalfa said he wanted to discuss with Lyon his background and relationship with the mayor before weighing in one way or the other. However, he said that residents involved in bygone political campaigns shouldn't be banished forever from serving on boards, such as the Planning Commission.
"How much time has to lapse before someone can participate?" he said. "I don't think you should be branded for life for political activity," LaMalfa said.
In an interview, Lyon said he hasn't worked for Becker since August 2011, when he left the mayor's re-election campaign to take the post at the Democratic Party.
"I have a strong relationship with Ralph Becker," he said. "I also have relationships with people on the council."
Working in the mayor's office brought insights into the types of decisions the Planning Commission faces, Lyon said.
"In the Planning Commission you want people involved in the community who know people and what's going on," he said. "That's what makes me a good candidate."
Nominees for unpaid positions on city boards and commissions usually fly through City Council approval, said Chairman Charlie Luke. But Lyon's relationship to the mayor makes his nomination different.
"If I were in [Becker's] position, I wouldn't have appointed my campaign manager," Luke said. "It raises the question of whether [the mayor] has too much influence over an impartial policy-making group."
Like his colleagues, Luke said Lyon is otherwise qualified for the position.
"This puts a lot of scrutiny on Matt, which is unfair," Luke said. "But the question is not about him it's about the relationship."