I remember Pee-wee hosted 'Saturday Night Live' later that year - in his opening monologue he mentions the movie, to cheers, saying "it did pretty wellbut not as good as [snarky voice] 'Back to the FUU-ture'!!! Oh, Michael J. Fox! Oh, [raspberry]!!!"

Budget was 7 million.

It made 40 million, and you'd have to guess that back in the mid 80's the budget was WAY under that. So it must've been a success.
That said the movie dropped and most of the population didn't even know who PeeWee was. So, he kinda hit the ground running that makes it even more impressive.
i was a kid, and we didn't catch it until home video The TV trailers seemed way too strange to check it out in the theater, but me and my siblings were probably shocked at how enjoyable it was, once we did finally watch it.

In the book 'Burton on Burton' He claims is was listed many times in 'worse movie of the year' polls and reviews at the time were poor. Since then it's all changed.

I don't recall it being rated one of the worst movies of the year. I remember some mixed and a couple of reviews being of the wth is this variety lol. But..then again, movie reviews back by all the critics were not as easily accessible.
I recall watching Siskel and Ebert on PBS, Rex Reed, Gene Shaliton and Lenoard Maltin all the time. There were a couple of others, but there was a show that I used to watch just here and there that gave a short quick review of movies.
I'm just not recalling PWBA being rated as the worse of the year. I also pulled this off of Wiki
Pee-wee's Big Adventure received generally positive reviews at the time of the film's release,[14] before eventually developing into a cult film.[14] Based on 36 reviews collected by Rotten Tomatoes, Pee-wee's Big Adventure has a 92% overall approval rating.[15] By comparison Metacritic calculated an average score of 47 from 13 reviews collected.[16] The film was nominated with a Young Artist Award for Best Family Motion Picture (Comedy or Musical).[17]
Christopher Null gave positive feedback, calling it "Burton's strangest film."[18] Variety compared Paul Reubens to Charlie Chaplin and Buster Keaton,[19] while Empire called the film "a one-comic masterpiece" and "a dazzling debut" for Burton.[20] Stephanie Zacharek of Salon.com explained "Everything about Pee-wee's Big Adventure, from its toy-box colors to its superb, hyper-animated Danny Elfman score to the butch-waxed hairdo and wooden-puppet walk of its star and mastermind is pure pleasure."[21] Burton was offered the opportunity to direct Big Top Pee-wee,[22] but had no interest and was already working on his own pet project, Beetlejuice. Positive reviews of Beetlejuice and the financial success of Pee-wee's Big Adventure prompted Warner Bros. to offer Burton the director's helm for Batman.[23]
Roger Ebert never officially reviewed Pee-Wee's Big Adventure but in 1987 it topped his list of Guilty Pleasures and he did mention it in his review of Big Top Pee-wee saying the sequel was not as magical as the first.[24] The second paragraph of that review contrasted the two films explaining what he liked in Big Adventure that he didn't like in Big Top.

Huge financial hit, but at the time rated one of the worse films of the year.
Now it's considered one of the best lol (same thing happened with beetlejuice)

Or did it just become more popular later on ?. Anyone know how popular is was in the UK when it was released?. I was only four years old when I first saw it in 1988. My parents rented a video of it.
It was ranked 19th for 1985 in the U.S. which isn't bad for a comedy. I don't see any figures for international box office so I'm not even sure that it was released theatrically worldwide.