I like her songs, but she always ends the verse with a "Lalalalaa"
Roads? Where we're going we don't need roads

But what about that same episode?
"Stephaniiiie knows all the chords."
That was it. Where was that going to go? All Phoebe knew about Stephanie was that she was a professional guitarist and that she knew all the chords.
In season 5's The One With the Kips, there's a few seconds where all Phoebe is doing is singing "la la la" and whistling.
And in season 3, she sings "Jingle Bitch" about the woman who took "Smelly Cat" for a commercial jingle.
One more example is in season 7 when Phoebe is sampling songs to Monica that she wanted to play at the wedding. In her defense in "First Time I Met Chandler," she says that part of the songs goes on. But all the other ones, "Addicted to Pork," "Whenever I Get Married," and then the two she sings outside of their bedroom door, it sounded like she was making up lines on the spot.
"Until I was 25, I thought the only response to 'I love you' was 'Oh, crap.'" -Chandler Bing

If you don't like the term messed up. Fine. I'll change to "I think it was a waste of time in a
Friends
episode." Why the song was full length is moot to that critique.

If you're gonna pretend to cut her hair, at least put some scissors in your hand!

But it wasn't a "mess up" like the Joey/Rachel thing, which had an impact on the show's story arc, while this did not. This was a couple of minutes where they wanted to showcase a well-known singer, and it wasn't relevant to the plot. This is like a joke you didn't find funny.

It was a mess-up for me. I think it was too long, and don't have the patience for sitting through it. When you're not a fan of something,
to you
, it's not good.
Like how many fans HATED Joey x Rachel. But the writers still felt they needed to have that story-line. I am sure they had reasons for that too. Even though Jennifer and Matt didn't care it.
Everything on this show was done with a reason. But fans still hate some stuff -the reasoning behind it is moot. You hate a plot, character choice, character, story line, etc. The creators know what they have in mind when making the episodes. But some fans won't always care for elements of it.
I think it was overkill -In the name and plot of the episode. We know Phoebe's plight. She wants to sing and play and CP. But now a professional comes in, which causes envy and intimidation, especially when everyone responds better to her. So Phoebe gets the shaft. But singing an ENTIRE song just seemed to be off pacing wise. Maybe have her start singing + playing, to show she's good. Then pan over to show the reactions of Phoebe and Rachel, then fade out of the scene to more actual story.

If you're gonna pretend to cut her hair, at least put some scissors in your hand!

Ok. I'm just pointing out that they had a well-known singer/musician as a guest to play music and sing, so of course it's going to go on longer than Phoebe's typical offerings. They didn't "mess up".

I still thought it was too much.
Everybody Loves Raymond
had Kristi Yamaguchi as a guest, but we didn't spend 5+ min. watching her skate.
So I do think her singing went on too long for the episode. Now if it was popular, then great. I wasn't watching Friends when this episode came out. So I don't know if it was praised highly for that scene in the 90s.
But myself, and others that I read, are in agreement with one another anyway as fans that the singing went on too long.

If you're gonna pretend to cut her hair, at least put some scissors in your hand!

They did that because they had Chrissie Hynde of The Pretenders as a guest.

Yeah. They messed up when they had Stephanie do that in season 2. She starts working at Central Perk in replacement of Phoebe. And I recall her singing for so long that I had to fast-forward through that crap.

If you're gonna pretend to cut her hair, at least put some scissors in your hand!

I think we can assume she wrote all her songs as full songs but we only hears snippets. Since a song is approximately about 3 minutes in length and a show is approximately 22 minutes of content, it wouldn't make sense for a whole song to be played out.

Consider the daffodil. And while you're doing that I'll be over here looking through your stuff.