The sentence

Je pense cela trop surpeuplé.

is not idiomatic and doesn’t translate “a place is overpopulated” but something like “I think this place too overpopulated”.

One way to improve it can be:

Je pense que c’est trop surpeuplé.

or, replacing penser by trouver:

Je trouve cela trop surpeuplé.

Cela is however odd to refer to a place so the next sentence works better:

Cet endroit, je le trouve trop surpeuplé.

Trop surpeuplé might be frowned upon but suggests that some level of overpopulation would be acceptable for you but in that case, it is beyond that point.

A more recent usage of trop could also lead to an opposite meaning: this place is is extremely overcrowded and that’s really cool.

On the other hand, there is no way to fix the incorrect :

Je pense cela trop surpeuplait.

if only because the verb surpeupler is very rare and is missing here both a realistic subject and a complement.